id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
270430189,MDU6SXNzdWUyNzA0MzAxODk=,1680,Coordinates get transformed to variables,806256,closed,0,,,7,2017-11-01T19:51:34Z,2017-11-08T21:37:04Z,2017-11-08T21:37:04Z,NONE,,,,"#### Code Sample, a copy-pastable example if possible

To get the data to reproduce the example: `aws s3 sync s3://olgabot-maca/xarray-coordinates-to-variables/ .` or download [this `.tar.gz` file](https://s3-us-west-2.amazonaws.com/olgabot-maca/xarray-coordinates-to-variables/olga_xarray-dev_subset_scrubbed.tar.gz).

[gist with code](https://gist.github.com/76892304d6285d5450f1d6014928fbe8)

#### Problem description

When I create a dataset, I set a bunch of metadata as `coordinates`, not `variables`:

![screen shot 2017-11-01 at 12 45 39 pm](https://user-images.githubusercontent.com/806256/32294155-9751934c-bf02-11e7-8c26-eeaec7dcfa5b.png)

But when I reload that exact same dataset with `xr.open_dataset`, the `coordinates` have be moved into `variables`!!

![screen shot 2017-11-01 at 12 45 54 pm](https://user-images.githubusercontent.com/806256/32294213-bba33ad4-bf02-11e7-9c3b-e147c8998847.png)

Interestingly, I think this is happening upon dataset creation because `ds.variables` already has the erroneous metadata:

![screen shot 2017-11-01 at 12 47 20 pm](https://user-images.githubusercontent.com/806256/32294247-d3b4b7c4-bf02-11e7-8b88-822d0513e33e.png)



#### Expected Output

I expected the coordinates and variables to be consistent between opening and closing the file

#### Output of ``xr.show_versions()``



<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-97-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

xarray: 0.10.0rc1-2-gf83361c
pandas: 0.20.3
numpy: 1.13.3
scipy: 0.19.1
netCDF4: None
h5netcdf: None
Nio: None
bottleneck: None
cyordereddict: None
dask: None
matplotlib: None
cartopy: None
seaborn: None
setuptools: 36.5.0.post20170921
pip: 9.0.1
conda: None
pytest: None
IPython: 6.1.0
sphinx: None

</details>
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1680/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue
266320445,MDU6SXNzdWUyNjYzMjA0NDU=,1638,Unicode strings unexpectedly transformed to byte strings upon `open_dataset`,806256,closed,0,,,7,2017-10-18T00:16:38Z,2017-11-01T19:51:55Z,2017-10-27T23:36:07Z,NONE,,,,"When I first create the dataset, all the metadata is stored as unicode strings (yay!):

```
<xarray.Dataset>
Dimensions:                       (cell: 53760, gene: 23438)
Coordinates:
  * gene                          (gene) object '0610005C13Rik' ...
    Uniquely mapped reads number  (cell) int64 1017682 634557 941828 1392029 ...
    Number of input reads         (cell) int64 1229254 730274 1075370 ...
    EXP_ID                        (cell) <U29 '170925_A00111_0066_AH3TKNDMXX' ...
    TAXON                         (cell) <U3 'mus' 'mus' 'mus' 'mus' 'mus' ...
    WELL_MAPPING                  (cell) <U9 'B000126' 'B000126' 'B000126' ...
    Lysis Plate Batch             (cell) <U32 '20' '20' '20' '20' '20' '20' ...
    dNTP.batch                    (cell) <U38 '457912' '457912' '457912' ...
    oligodT.order.no              (cell) <U32 '6/23/17 12757296' ...
    plate.type                    (cell) <U32 'Biorad HSP3901' ...
    preparation.site              (cell) <U32 'Biohub' 'Biohub' 'Biohub' ...
    date.prepared                 (cell) <U32 '07-06-17' '07-06-17' ...
    date.sorted                   (cell) <U6 '170707' '170707' '170707' ...
    tissue                        (cell) <U13 'Skin' 'Skin' 'Skin' 'Skin' ...
    subtissue                     (cell) <U32 'nan' 'nan' 'nan' 'nan' 'nan' ...
    mouse.id                      (cell) <U13 '3_39_F' '3_39_F' '3_39_F' ...
    FACS.selection                (cell) <U52 'Multiple' 'Multiple' ...
    nozzle.size                   (cell) <U32 '100' '100' '100' '100' '100' ...
    FACS.instument                (cell) <U32 'Sony SIM1' 'Sony SIM1' ...
    Experiment ID                 (cell) <U32 'exp22' 'exp22' 'exp22' ...
    Columns sorted                (cell) float64 nan nan nan nan nan nan nan ...
    Double check                  (cell) float64 nan nan nan nan nan nan nan ...
    Plate                         (cell) <U32 '1' '1' '1' '1' '1' '1' '1' ...
    Location                      (cell) <U32 'MACA20_3' 'MACA20_3' ...
    Comments                      (cell) <U32 'nan' 'nan' 'nan' 'nan' 'nan' ...
    mouse.age                     (cell) <U1 '3' '3' '3' '3' '3' '3' '3' '3' ...
    mouse.number                  (cell) <U32 '39' '39' '39' '39' '39' '39' ...
    mouse.sex                     (cell) <U1 'F' 'F' 'F' 'F' 'F' 'F' 'F' 'F' ...
  * cell                          (cell) object 'A17-B000126-3_39_F-1-1' ...
Data variables:
    counts                        (cell, gene) int64 0 0 0 0 442 0 0 0 0 0 0 ...
    log2                          (cell, gene) float64 0.0 0.0 0.0 0.0 8.791 ...
    log10                         (cell, gene) float64 0.0 0.0 0.0 0.0 2.646 ...
```

but then when I save using `to_netcdf` using the default arguments, then `xr.open_dataset` on the same dataset using default arguments, all of them get converted to byte strings:

```
<xarray.Dataset>
Dimensions:                       (cell: 53760, gene: 23438)
Coordinates:
  * cell                          (cell) |S24 b'A17-B000126-3_39_F-1-1' ...
  * gene                          (gene) |S22 b'0610005C13Rik' ...
Data variables:
    counts                        (cell, gene) int32 0 0 0 0 442 0 0 0 0 0 0 ...
    log2                          (cell, gene) float64 0.0 0.0 0.0 0.0 8.791 ...
    log10                         (cell, gene) float64 0.0 0.0 0.0 0.0 2.646 ...
    FACS.selection                (cell) |S52 b'Multiple' b'Multiple' ...
    dNTP.batch                    (cell) |S38 b'457912' b'457912' b'457912' ...
    EXP_ID                        (cell) |S29 b'170925_A00111_0066_AH3TKNDMXX' ...
    subtissue                     (cell) |S19 b'nan' b'nan' b'nan' b'nan' ...
    oligodT.order.no              (cell) |S17 b'6/23/17 12757296' ...
    plate.type                    (cell) |S14 b'Biorad HSP3901' ...
    tissue                        (cell) |S13 b'Skin' b'Skin' b'Skin' ...
    mouse.id                      (cell) |S13 b'3_39_F' b'3_39_F' b'3_39_F' ...
    FACS.instument                (cell) |S13 b'Sony SIM1' b'Sony SIM1' ...
    Comments                      (cell) |S11 b'nan' b'nan' b'nan' b'nan' ...
    WELL_MAPPING                  (cell) |S9 b'B000126' b'B000126' ...
    date.prepared                 (cell) |S9 b'07-06-17' b'07-06-17' ...
    Location                      (cell) |S9 b'MACA20_3' b'MACA20_3' ...
    preparation.site              (cell) |S8 b'Biohub' b'Biohub' b'Biohub' ...
    date.sorted                   (cell) |S6 b'170707' b'170707' b'170707' ...
    Experiment ID                 (cell) |S6 b'exp22' b'exp22' b'exp22' ...
    TAXON                         (cell) |S3 b'mus' b'mus' b'mus' b'mus' ...
    Lysis Plate Batch             (cell) |S3 b'20' b'20' b'20' b'20' b'20' ...
    nozzle.size                   (cell) |S3 b'100' b'100' b'100' b'100' ...
    Plate                         (cell) |S3 b'1' b'1' b'1' b'1' b'1' b'1' ...
    mouse.number                  (cell) |S3 b'39' b'39' b'39' b'39' b'39' ...
    Uniquely mapped reads number  (cell) int32 1017682 634557 941828 1392029 ...
    Number of input reads         (cell) int32 1229254 730274 1075370 ...
    Columns sorted                (cell) float64 nan nan nan nan nan nan nan ...
    Double check                  (cell) float64 nan nan nan nan nan nan nan ...
    mouse.age                     (cell) |S1 b'3' b'3' b'3' b'3' b'3' b'3' ...
    mouse.sex                     (cell) |S1 b'F' b'F' b'F' b'F' b'F' b'F' ...
```

So then things I expect like selecting on gene, e.g. `ds.sel(gene=""Ins1"")` don't work unless they're byte strings, i.e. `ds.sel(gene=b""Ins1"")` works just fine. 

Do you know why this may be happening?","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1638/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue