False negative on SettingWithCopyWarning
This is pandas 0.15.2

```
In [1]: from pandas import *

In [2]: df = DataFrame({"A" : [1,2,3,4,5], "B": [3.1, 4, 5, 6, 7]})

In [3]: df[["A", "B"]]["A"] = 5  # no warning - bug?!

In [4]: df
Out[4]:
A    B
0  1  3.1
1  2  4.0
2  3  5.0
3  4  6.0
4  5  7.0

In [5]: df[["A"]]["A"] = 5
C:\Anaconda\Scripts\ipython-script.py:1: SettingWithCopyWarning:
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
if __name__ == '__main__':
```
