Dataframe.unstack().stack(0) erroneously changes data if indices not initially sorted
#### Code Sample, a copy-pastable example if possible

```python
>>> import pandas as pd
>>> # Notice that the columns are not sorted below.
>>> df = pd.DataFrame(data=[[0,1],[2,3],[4,5],[6,7]],index = pd.MultiIndex.from_product([['a','b'],['A','B']]),columns=['d','c'])
>>> # The value of the element with indices 'b', 'B', and 'd'
>>> df.loc[('b','B'),'d']
6
>>> # The value of that *same* element now.
>>> df.unstack().stack(0).loc[('b','d'),'B']
7
>>> # What went wrong?
>>> df
     d  c
a A  0  1
  B  2  3
b A  4  5
  B  6  7
>>> # During some step, the indices got sorted but the values did not follow.
>>> df.unstack().stack(0)
     A  B
a d  1  3
  c  0  2
b d  5  7
  c  4  6
```
#### Problem description

With `MultiIndex`ed `DataFrames`, it becomes convenient to `unstack(level)` and `stack(level)` your DataFrame until it has the indices you need to do what you want to do. These methods will sort your indices or levels if they were not sorted to begin with. 

However, apparently I have discovered a case where the indices got sorted, but the values did not follow, resulting in the "shuffling" you see above.

#### Expected Output

The expected behavior is that these operations should not result in data scrambling / shuffling; a complete set of indices (like {'b','B','d'}) should always refer to the same value (in this case, 6).

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.1
pytest: 3.0.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 5.3.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
