sort_index behavior differs for the same DataFrame?
I feel like I've encountered a bug. In the following scenario, the first `sort_index` call behaves as expected, but the second does not. Does someone know what the difference is here?

```
In [1]: import pandas as pd

In [2]: pd.__version__
Out[2]: '0.15.2'

In [3]: tuples = [(' foo', 'bar'), ('foo', 'bar'), (' foo ()', 'bar')]

In [4]: cols = pd.MultiIndex.from_tuples(tuples)

In [5]: df = pd.DataFrame(index=cols, data={'baz': [0, 1, 2]})

In [6]: df
Out[6]: 
             baz
 foo    bar    0
foo     bar    1
 foo () bar    2

In [7]: df.sort_index()
Out[7]: 
             baz
 foo    bar    0
 foo () bar    2
foo     bar    1

In [8]: tuples = [(' foo', 'bar'), ('foo', 'bar')]

In [9]: cols = pd.MultiIndex.from_tuples(tuples)

In [10]: df = pd.DataFrame(index=cols, data={'baz': [0, 1]})

In [11]: df
Out[11]: 
          baz
 foo bar    0
foo  bar    1

In [12]: df.ix[(' foo ()', 'bar'), 'baz'] = 2

In [13]: df
Out[13]: 
             baz
 foo    bar    0
foo     bar    1
 foo () bar    2

In [14]: df.sort_index()
Out[14]: 
             baz
 foo    bar    0
foo     bar    1
 foo () bar    2
```
