FIX groupby name without multiindex GH4014
Fixes #4014.

Just checked the fix, thanks a lot. I should get into some pandas dev, too. What's the best way to get started?

FYI, the fix is for GH4014, in the comment on 1418 in pandas/tests/test_groupby.py you specified GH4104 (as in the heading here). Just for future referencing...
 Hehe thanks! (I'll fix it thanks, apparently I can't type.)

Perhaps a good way to get started is to check out the ["Good as first PR" tag](https://github.com/pydata/pandas/issues?direction=desc&labels=Good+as+first+PR&page=1&sort=updated&state=open) :)
