BUG: groupby resample different results with .agg() vs .mean() 
- [x] closes #33548
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @jalmaguer! 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 2020-12-21 05:42:45 UTC some  CI Checks errors @jreback I made some commits a few days ago since your last comments so I just wanted to bump this pull request so that it doesn't get lost and forgotten about.  can you merge master (been lots of changes in groupby recently) > can you merge master (been lots of changes in groupby recently)

@jreback Sure no problem.  Just merged it.  Looks like it should be good to go. @jreback Just bumping this so it doesn't get forgotten ok we had some patches which might have fixed this

can u merge master (and also test if your added tests pass w/o the patch) @jreback Just merged master.  Looks like my added tests do not pass without the patch. thanks @jalmaguer  @meeseeksdev backport 1.2.x