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.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14333?src=pr) is 85.26% (diff: 95.83%)

> Merging [#14333](https://codecov.io/gh/pydata/pandas/pull/14333?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14333   diff @@
==========================================
  Files           140        140          
  Lines         50630      50639     +9   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43169      43177     +8   
- Misses         7461       7462     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [6dcc238...33eb725](https://codecov.io/gh/pydata/pandas/compare/6dcc23862b6b60ce2a67436b4a278fbe4c05490f...33eb725c39e38b89c46042848dce703de78a6b52?src=pr)
 whatsnew moved to 0.20.0 as @jreback requested
 Looks good to me
 Thanks for the feedback @jreback, @jorisvandenbossche, and @TomAugspurger.  Writing the docstrings and working through the type signatures sure helped me better understand the logic I was refactoring!
 @TomAugspurger @jorisvandenbossche @jreback I've noticed that there's now a 0.19.1 whatsnew file. Should I move the whatsnew entry there?
 Yes, as it seems a straightforward bug fix, you can put it in 0.19.1
 @jorisvandenbossche Hmm, I rebased my branch on upstream/master so that I have a copy of the 0.19.1 whatsnew file to work with. After making the change I did a force push which successfully updated the branch in my fork (https://github.com/jmmease/pandas/tree/bug_14327), but it didn't update this pull request.

Should I have merged upstream/master instead of rebasing?
 Well, something is going wrong since we moved the repo from the pydata org to the pandas-dev org. PRs that were already open before seem to have this problem.

@wesm @TomAugspurger @jreback anybody experience with this problem, or an idea what could be going on?
 @jmmease lgtm. just some doc consistency changes. ping on green.
 @jreback Thanks, I'll make these updates this evening. Do you have any guidance on how I should proceed since this PR didn't update with my rebase+force push from yesterday? (See comment from @jorisvandenbossche above) I could try again with a merge instead of rebase or I could open a new PR and reference this one.
 On the short term, opening as a new PR is probably the easiest (if you close this one, you can just create a new one from the same branch)
 PR reopened in #14428
