sort_index(inplace = True) produces SettingWithCopyWarning
When I am trying to do `rows.sort_index(inplace = True)` I get SettingWithCopyWarning:
`A value is trying to be set on a copy of a slice from a DataFrame`

`rows = rows.sort_index()` can be used as a workaround to avoid the warning.

Pandas version 0.17.0 installed by pip. 
