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
301856320,MDExOlB1bGxSZXF1ZXN0MTcyNTY4Njcx,1953,Fix maybe_promote,1997005,closed,0,,,3,2018-03-02T18:11:49Z,2018-08-20T16:35:50Z,2018-08-20T16:31:16Z,CONTRIBUTOR,,0,pydata/xarray/pulls/1953,"With tests for every possible dtype:

(numpy docs say `biufcmMOSUV` only)

```python
for letter in string.ascii_letters:
    try:
        print(letter, np.dtype(letter))
    except TypeError as exc:
        pass
```

 - [x] Closes #1952
 - [x] Tests added (for all bug fixes or enhancements)
 - [x] Tests passed (for all non-documentation changes)
 - [ ] Fully documented, including `whats-new.rst` for all changes and `api.rst` for new API (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1953/reactions"", ""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull
301795163,MDU6SXNzdWUzMDE3OTUxNjM=,1952,xarray.merge exception : invalid type promotion,1997005,closed,0,,,4,2018-03-02T15:02:43Z,2018-08-20T16:31:16Z,2018-08-20T16:31:16Z,CONTRIBUTOR,,,,"#### Code Sample

```python
import xarray
import pandas
import numpy

array1 = xarray.DataArray(
    [numpy.timedelta64('NaT')],
    dims=['time'],
    coords={'time': pandas.to_datetime(['2018-01-01'])},
    name='foo'
)

array2 = xarray.DataArray(
    [numpy.timedelta64(30, 's')],
    dims=['time'],
    coords={'time': pandas.to_datetime(['2018-01-02'])},
    name='foo'
)

xarray.merge([array1, array2])
```
#### Problem description

Merging arrays with identical dtypes should work ...

There's some issue with the `NaT` being interpreted as float64, so that `xarray.core.dtypes.result_type` thinks that those 2 arrays are not compatible..

It works with `xarray==0.10.0` and fails at `xarray==0.10.1`.
I've pin-pointed the issue to commit 2aa5b8a5c0945935.

Work-around in the mean time :
```python
xarray.merge([array1.astype(float), array2.astype(float)]).astype('timedelta64')
```

#### Expected Output

```python

# expected :
xarray.DataArray(
    [numpy.timedelta64('NaT'), numpy.timedelta64(30, 's')],
    dims=['time'],
    coords={'time': pandas.to_datetime(['2018-01-01', '2018-01-02'])},
    name='foo'
)
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-142-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8
LOCALE: fr_FR.UTF-8

xarray: 0.10.0+dev60.g2aa5b8a
pandas: 0.22.0
numpy: 1.14.0
scipy: None
netCDF4: 1.3.1
h5netcdf: None
h5py: None
Nio: None
zarr: None
bottleneck: None
cyordereddict: None
dask: None
distributed: None
matplotlib: 2.1.1
cartopy: None
seaborn: None
setuptools: 38.4.0
pip: 9.0.1
conda: None
pytest: 3.3.2
IPython: 6.2.1
sphinx: None
</details>


Thanks !","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1952/reactions"", ""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue
220241754,MDU6SXNzdWUyMjAyNDE3NTQ=,1359,Bug: open_dataarray does not pass properly its parameters to open_dataset,1997005,closed,0,,2444330,1,2017-04-07T15:06:50Z,2017-04-12T22:28:35Z,2017-04-12T22:28:35Z,CONTRIBUTOR,,,,"Hi,

There's now a `autoclose` parameter to the `open_dataset` function.
But `open_dataarray` uses positional arguments only, so now the arguments are no longer at their place, and `engine`, for instance, can't be specified.

Thanks !","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1359/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue