issues: 417412201

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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()

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 
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   completed 13221727 issue

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