Pandas sometimes converts multi-indexes to indexes when indexing with all elements
these is done with Pandas 0.12
I create a DataFrame with a MultiIndex

``` Python
    >>> h = pd.DataFrame([{'a':1, 'b':1, 'c':1}, {'a':1, 'b':2, 'c':1}])
    >>> h.set_index(['a', 'b'], inplace=True)
```

If I take a list of elements (here only a list with one) of a column, I get back a Series with a MultiIndex:

```
    >>> h['c'][[(1,1)]].index
    MultiIndex
    [(1, 1)]
```

However, if I happen to include all elements in the indexing list, then somehow pandas convertes my MultiIndex to an Index:

``` Python
    >>> h['c'][[(1,1), (1,2)]].index
    Index([(1, 1), (1, 2)], dtype=object)
```

First indexing and taking the column of the DataFrame preserves the MultiIndex:

``` Python
    >>> h.ix[[(1,1), (1,2)]]['c'].index
    MultiIndex
    [(1, 1), (1, 2)]
```

I noticed this while trying to unstack a Series that I indexed with a MultiIndex list and for a certain case the list included all elements leading to this exception:

``` Python
    >>> h['c'][[(1,1), (1,2)]].unstack()
    AttributeError                            Traceback (most recent call last)
    <ipython-input-916-d897801d99a0> in <module>()
    ----> 1 h['c'][[(1,1), (1,2)]].unstack()

    C:\x64\Python27\lib\site-packages\pandas\core\series.pyc in unstack(self, level)
       2437         """
       2438         from pandas.core.reshape import unstack
    -> 2439         return unstack(self, level)
       2440 
       2441     #----------------------------------------------------------------------

    C:\x64\Python27\lib\site-packages\pandas\core\reshape.pyc in unstack(obj, level)
        417             return obj.T.stack(dropna=False)
        418     else:
    --> 419         unstacker = _Unstacker(obj.values, obj.index, level=level)
        420         return unstacker.get_result()
        421 

    C:\x64\Python27\lib\site-packages\pandas\core\reshape.pyc in __init__(self, values, index, level, value_columns)
         72         self.level = self.index._get_level_number(level)
         73 
    ---> 74         levels = index.levels
         75         labels = index.labels
         76         def _make_index(lev,lab):

    AttributeError: 'Index' object has no attribute 'levels'
```
