BUG: index.max() not matching index.levels[0].max() for multiindex on sliced dataframe
Small example:

``` python
df = pd.DataFrame(index=pd.MultiIndex.from_product([['A','B','C'],['a','b']])
```

Then:

``` python
df.loc[:'B'].index.max()
```

gives `('B', 'b')` as expected.
But:

``` python
df.loc[:'B'].index.levels[0].max()
```

gives `'C'` in conflict with the previous result.
#### Expected Functionality

The max index of each level matches the max index of the full multiindex
(Apologies if this is not actually intended functionality)
