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

This PR is a continuation of #14333. See discussion there for explanation of why this new PR was needed.

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14428?src=pr) is 85.26% (diff: 96.29%)

> Merging [#14428](https://codecov.io/gh/pandas-dev/pandas/pull/14428?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14428   diff @@
==========================================
  Files           140        140          
  Lines         50639      50649    +10   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43177      43186     +9   
- Misses         7462       7463     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [286b9b9...9694755](https://codecov.io/gh/pandas-dev/pandas/compare/286b9b94d97fb2256d228a8970e92eee2ac3078a...9694755e1dea4be8dbcbb3484abd22fdbb90c4a7?src=pr)
 @jreback it's green
 @jmmease can you make the doc-string updates.
 @jreback Made documentation updates and then rebased to resolve conflict in whatsnew 0.19.1.
Let me know if there's anything else!
 lgtm

ping on green
 @jreback green 
 @jmmease Thanks!
