BUG: Fix groupby methods to include *args and **kwds if applicable.
Merge pull request #8758 from dstephens99/issue8733

BUG: Fix groupby methods to include *args and **kwds if applicable.
DOC: clean-up v0.15.1 whatsnew file
Merge pull request #8755 from jorisvandenbossche/doc-fixes2

DOC: clean-up v0.15.1 whatsnew file
BUG: concat of series of dtype category converting to object dtype (GH8641)