Fix passing args in groupby plot (GH11805)
Closes https://github.com/pydata/pandas/issues/11805

So probably all groupby plot calls that use kwargs are broken in 0.17.x. 
@shoyer it was only a very small change actually!

Still need to add tests.

Good catch! I guess we had very poor coverage for groupby plots previously.
 Very poor coverage indeed .. 
Added tests + whatsnew notice.
 lgtm
