implemented fix for groupby date bug, #11324
This is a fix for issue #11324 in which grouping when datetime fields are involved raises an exceptoin

pls add a whatsnew comment (bug fix section), and squash.
 Added comments to test, made an entry in v0.17.1.txt, and squashed commits.
 merged via https://github.com/pydata/pandas/commit/5df693fafa7eedf732d414cc9eb8310b5391dfae

thanks!
