`values=['c']` returns `aggfunc` on all columns, instead of just the passed column
- [ ] closes #29080


Hello @quanghm! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-11-12 04:58:12 UTC @quanghm can you add tests for the bug this fixes > 
> 
> @quanghm can you add tests for the bug this fixes

@jbrockmendel I'm not sure how to add the tests. Can you point to some links? Thanks. > I'm not sure how to add the tests. Can you point to some links? 

Take a look in pandas/tests/groupby/ for examples @quanghm is this still active? Can you add tests? > 
> 
> @quanghm is this still active? Can you add tests?

Sorry, has been caught at work lately. I'll write the test over the weekend. @quanghm looks like CI is failing - can you take a look? closing as stale. if you'd like to continue, pls ping. this needs tests.