series.to_xarray() fails when MultiIndex not sorted in xarray 0.15.1

Summary

It seems that series.to_xarray() fails (returns incorrect data) in xarray 0.15.1 when the dataframe's MultiIndex dimensions are not sorted

Demonstration

xarray should be able to handle MultiIndices with unsorted dimensions. Using a fresh conda environment with xarray 0.14.1:

```python $ conda run -n py37xr14 python test.py

df alpha B A num 0 1 4 1 2 5 2 3 6

df.stack('alpha') num alpha 0 B 1 A 4 1 B 2 A 5 2 B 3 A 6 dtype: int64

df.stack('alpha').to_xarray() <xarray.DataArray (num: 3, alpha: 2)> array([[1, 4], [2, 5], [3, 6]]) Coordinates: * num (num) int64 0 1 2 * alpha (alpha) object 'B' 'A' ```

This fails in xarray 0.15.1 - note the data is not merely reordered - the data in column 'B' now has the incorrect values 4, 5, 6 rather than 1, 2, 3:

```python $ conda run -n py37xr15 python test.py

df alpha B A num 0 1 4 1 2 5 2 3 6

df.stack('alpha') num alpha 0 B 1 A 4 1 B 2 A 5 2 B 3 A 6 dtype: int64

df.stack('alpha').to_xarray() <xarray.DataArray (num: 3, alpha: 2)> array([[4, 1], [5, 2], [6, 3]]) Coordinates: * num (num) int64 0 1 2 * alpha (alpha) object 'B' 'A' ```

Test setup & environment info

contents of test.py

```python import pandas as pd df = pd.DataFrame({'B': [1, 2, 3], 'A': [4, 5, 6]}) df = df.rename_axis('num').rename_axis('alpha', axis=1) print(">>> df") print(df) print("\n>>> df.stack('alpha')") print(df.stack('alpha')) print("\n>>> df.stack('alpha').to_xarray()") print(df.stack('alpha').to_xarray()) ```

packages in py37xr14 environment

```bash $ conda list -n py37xr14 # packages in environment at /Users/delgadom/miniconda3/envs/py37xr14: # # Name Version Build Channel ca-certificates 2020.4.5.1 hecc5488_0 conda-forge certifi 2020.4.5.1 py37hc8dfbb8_0 conda-forge libblas 3.8.0 16_openblas conda-forge libcblas 3.8.0 16_openblas conda-forge libcxx 9.0.1 2 conda-forge libffi 3.2.1 h4a8c4bd_1007 conda-forge libgfortran 4.0.0 2 conda-forge liblapack 3.8.0 16_openblas conda-forge libopenblas 0.3.9 h3d69b6c_0 conda-forge llvm-openmp 9.0.1 h28b9765_2 conda-forge ncurses 6.1 h0a44026_1002 conda-forge numpy 1.18.1 py37h7687784_1 conda-forge openssl 1.1.1f h0b31af3_0 conda-forge pandas 1.0.3 py37h94625e5_0 conda-forge pip 20.0.2 py_2 conda-forge python 3.7.6 h90870a6_5_cpython conda-forge python-dateutil 2.8.1 py_0 conda-forge python_abi 3.7 1_cp37m conda-forge pytz 2019.3 py_0 conda-forge readline 8.0 hcfe32e1_0 conda-forge setuptools 46.1.3 py37hc8dfbb8_0 conda-forge six 1.14.0 py_1 conda-forge sqlite 3.30.1 h93121df_0 conda-forge tk 8.6.10 hbbe82c9_0 conda-forge wheel 0.34.2 py_1 conda-forge xarray 0.14.1 py_1 conda-forge xz 5.2.5 h0b31af3_0 conda-forge zlib 1.2.11 h0b31af3_1006 conda-forge ```

packages in py37xr15 environment

```bash $ conda list -n py37xr15 # packages in environment at /Users/delgadom/miniconda3/envs/py37xr15: # # Name Version Build Channel ca-certificates 2020.4.5.1 hecc5488_0 conda-forge certifi 2020.4.5.1 py37hc8dfbb8_0 conda-forge libblas 3.8.0 16_openblas conda-forge libcblas 3.8.0 16_openblas conda-forge libcxx 9.0.1 2 conda-forge libffi 3.2.1 h4a8c4bd_1007 conda-forge libgfortran 4.0.0 2 conda-forge liblapack 3.8.0 16_openblas conda-forge libopenblas 0.3.9 h3d69b6c_0 conda-forge llvm-openmp 9.0.1 h28b9765_2 conda-forge ncurses 6.1 h0a44026_1002 conda-forge numpy 1.18.1 py37h7687784_1 conda-forge openssl 1.1.1f h0b31af3_0 conda-forge pandas 1.0.3 py37h94625e5_0 conda-forge pip 20.0.2 py_2 conda-forge python 3.7.6 h90870a6_5_cpython conda-forge python-dateutil 2.8.1 py_0 conda-forge python_abi 3.7 1_cp37m conda-forge pytz 2019.3 py_0 conda-forge readline 8.0 hcfe32e1_0 conda-forge setuptools 46.1.3 py37hc8dfbb8_0 conda-forge six 1.14.0 py_1 conda-forge sqlite 3.30.1 h93121df_0 conda-forge tk 8.6.10 hbbe82c9_0 conda-forge wheel 0.34.2 py_1 conda-forge xarray 0.15.1 py_0 conda-forge xz 5.2.5 h0b31af3_0 conda-forge zlib 1.2.11 h0b31af3_1006 conda-forge ```