BUG: properly handle a user function ingroupby that returns all scalars when
     grouping on a dataframe (GH5592) revisited
BUG: handle coercion of None for date/time like dtypes in a groupby result
Merge pull request #5675 from jreback/groupby_none

BUG: properly handle a user function ingroupby that returns all scalars  (GH5592)