Bug: Error when key-only Grouper is passed to groupby in a list (GH14334)
- [x] closes #14334
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

@jorisvandenbossche Is this Travis CI build failure something I need to worry about?
 That does not seem related, and I can also not reproduce it locally. I restarted the build.
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14342?src=pr) is 85.26% (diff: 100%)

> Merging [#14342](https://codecov.io/gh/pandas-dev/pandas/pull/14342?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14342   diff @@
==========================================
  Files           140        140          
  Lines         50640      50645     +5   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43176      43180     +4   
- Misses         7464       7465     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [794f792...5e96797](https://codecov.io/gh/pandas-dev/pandas/compare/794f79295484298525dbc8dd3b8ab251ad065e61...5e9679773630ab18d14f6d0ed4b61a1ba0f910f4?src=pr)
 @jreback rebased, moved whatsnew to 0.19.1, and made the requested updates
 @jreback multiple column `pd.Grouper` tests added
 thanks!
 @jreback @jorisvandenbossche I see that the PR is closed but it doesn't look like it was merged into master. Was there a problem or does it just take a while?
 @jmmease No, it is merged, just not through the github interface. You can see the merge commit in the "jreback closed this in 4852008 a day ago" message above. If you look at master history, you will also see a commit of you with the PR title as message
 @jorisvandenbossche Ahh, I see. Thanks
