Setting values on a DataFrame with tz aware DatetimeIndex fails
``` python

tdf = pd.DataFrame({'a': np.arange(10), 'b': np.arange(10) + 5}, 
                   index=pd.date_range('20160119 09:00', freq='5T', 
                                       periods=10, tz='US/Eastern'))
new_values = tdf.sample(2)['b'] + 100
tdf.loc[new_values.index, 'b'] = new_values.values
```

This fails with `KeyError: "['2016-01-19T14:00:00.000000000+0000' '2016-01-19T14:05:00.000000000+0000'] not in index`

This used to work. Not sure which version broke it.
