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
417412201,MDU6SXNzdWU0MTc0MTIyMDE=,2802,stack renames single dimensions instead of creating a MultiIndex,601177,closed,0,,,3,2019-03-05T17:24:54Z,2021-01-19T16:32:11Z,2021-01-19T16:32:11Z,NONE,,,,"#### Code Sample

```python
>>> da = xr.DataArray([0., 1., 2.])
>>> db = da.stack(z=da.dims)
>>> db
<xarray.DataArray (z: 3)>
array([0., 1., 2.])
Coordinates:
  * z        (z) int64 0 1 2
>>> db.unstack('z')
Traceback (most recent call last):

  File ""<ipython-input-127-ec66ebede9c5>"", line 1, in <module>
    db.unstack('z')

  File ""/usr/lib64/python3.6/site-packages/xarray/core/dataarray.py"", line 1248, in unstack
    ds = self._to_temp_dataset().unstack(dim)

  File ""/usr/lib64/python3.6/site-packages/xarray/core/dataset.py"", line 2315, in unstack
    raise ValueError('cannot unstack a dimension that does not have '

ValueError: cannot unstack a dimension that does not have a MultiIndex
```
#### Problem description
`stack` is not consistently creating a `MultiIndex`, but just renames a dimension in case there is just the single dimension to be stacked. This breaks code where the (number of) dimensions are (is) not known in advance.

#### Expected Output
`stack` should create a `MultiIndex` in all cases and a `stack`/`unstack` sequence should always return the original `DataArray`.

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

<details>

commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.19.23-gentoo
machine: x86_64
processor: Intel(R) Core(TM) i7-2620M CPU @ 2.70GHz
byteorder: little
LC_ALL: None
LANG: nl_BE.UTF-8
LOCALE: nl_BE.UTF-8

xarray: 0.10.8
pandas: 0.19.1
numpy: 1.14.5
scipy: 0.19.1
netCDF4: 1.3.1
h5netcdf: None
h5py: 2.9.0
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: None
dask: None
distributed: None
matplotlib: 2.2.2
cartopy: None
seaborn: None
setuptools: 40.6.3
pip: 9.0.1
conda: None
pytest: 3.10.1
IPython: 5.4.1
sphinx: 1.7.5
</details>
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396457448,MDU6SXNzdWUzOTY0NTc0NDg=,2658,Dataset character coordinates change to object upon use in Dataset,601177,closed,0,,,2,2019-01-07T11:50:41Z,2021-01-13T17:09:05Z,2021-01-13T17:09:05Z,NONE,,,,"#### Code Sample

```python
>>> import xarray as xr

>>> test = xr.Dataset(coords={'xy': ['x', 'y']})

>>> test
<xarray.Dataset>
Dimensions:  (xy: 2)
Coordinates:
  * xy       (xy) <U1 'x' 'y'  # NOTE '<U1' dtype
Data variables:
    *empty*

>>> test['test'] = xr.DataArray(np.array([0, 0]), dims=['xy'])

>>> test
<xarray.Dataset>
Dimensions:  (xy: 2)
Coordinates:
  * xy       (xy) object 'x' 'y'  # NOTE 'object' dtype
Data variables:
    test     (xy) int64 0 0
```
#### Problem description

The coordinate `dtype` changes from `<U1` to `object`.

#### Expected Output

The coordinate `dtype` should not change.

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

<details>
/usr/lib64/python3.6/site-packages/h5py/__init__.py:36: FutureWarning: Conversion of the second argument of issubdtype from `float` to `np.floating` is deprecated. In future, it will be treated as `np.float64 == np.dtype(float).type`.
  from ._conv import register_converters as _register_converters

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.14.83-gentoo
machine: x86_64
processor: Intel(R) Core(TM) i7-2620M CPU @ 2.70GHz
byteorder: little
LC_ALL: None
LANG: nl_BE.UTF-8
LOCALE: nl_BE.UTF-8

xarray: 0.10.8
pandas: 0.19.1
numpy: 1.14.5
scipy: 0.19.1
netCDF4: 1.3.1
h5netcdf: None
h5py: 2.7.1
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: None
dask: None
distributed: None
matplotlib: 2.2.2
cartopy: None
seaborn: None
setuptools: 36.7.2
pip: 9.0.1
conda: None
pytest: 3.2.2
IPython: 5.4.1
sphinx: 1.7.5
</details>
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