pydata/pandas — pull request #9177 by ledmonster (2014-12-31 16:16:37+00:00)
Title: BUG: Fix not to reindex on non-Categorical groups (GH9049)

Fixed the issue #9049.

*self._was_factor* is not appropriate to judge whether grouper is Categorical or not, because it can be "True" when we groupby indices (not columns). So, I added another flag *self._is_categorical* to judge Categorical state.

Also, I added a GroupBy test for MultiIndexed data, which was failed before this fix.