BUG: Ensure same index is returned for slow and fast path in groupby.apply
This _fixes_ the internal check to be consistent with the slow apply path

- [x] closes #31612
- [x] closes #14927
closes #13056 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @fjetter! 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 2020-05-27 12:48:36 UTC are any of the other issues you referenced closed by this? (aside from the ones you listed) is this a regression from 0.25.x? > are any of the other issues you referenced closed by this? (aside from the ones you listed)

The issues I referenced in the issue are already closed. I primarily referenced them to highlight this as a hotspot.

> is this a regression from 0.25.x

I believe this is not a regression and has been around for a while now. I'll check anyhow to confirm.

- [x] check if it a regression If this is not a regression, let's keep it for 1.1 then, I would say Agreed. also pls merge master I've found that if I disable fast_apply, then there is a test failure in tests.groupby.test_whitelist for `tshift`.  any idea why this doesnt affect that? can you merge master and ping on green and we can look sorry, was a bit swamped. Will rebase and address the comments @jreback I rebased but it seems there is a (probably unrelated) test failure on the azure pipelines with `test_tab_complete_*` tests. Is this a known issue? > there is a (probably unrelated) test failure on the azure pipelines with test_tab_complete_* tests. Is this a known issue?

It is unrelated, but not yet trouble-shot. > > there is a (probably unrelated) test failure on the azure pipelines with test_tab_complete_* tests. Is this a known issue?
> 
> It is unrelated, but not yet trouble-shot.

@fjetter #33567 has been fixed. can you merge upstream/master. I rebased on master and kept the changelog for 1.1.0 ping on green. Trying to rewrite the changelog. pretty difficult to actually list all tickets affected by this since most users don't go into depths that far.
There is also an umbrella issue which tracks these kind of things which we may be able to close now #13056 another opinion appreciated) > Trying to rewrite the changelog. pretty difficult to actually list all tickets affected by this since most users don't go into depths that far.
> There is also an umbrella issue which tracks these kind of things which we may be able to close now #13056 another opinion appreciated)

what you wrote is ok, the idea is really to just list the affected tickets so that users can click thru if desired. when we close an issue we want it referenced in the whatsnew and a test to confirm :-> thanks @fjetter very nice!