Bug: Grouping by index and column fails on DataFrame with single index (GH14327)
- [x] closes #14327
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Don't know if this is too late for 0.19.0 but I went ahead and added the whatsnew entry there for now.
