BUG: fix groupby.transform rename bug (#23461)

[DiegoAlbertoTorres]

• closes df.groupby(...).transform(func) breaks when func renames input df #23461
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @DiegoAlbertoTorres! Thanks for updating the PR.

• There are no PEP8 issues in the file pandas/core/groupby/generic.py !

• There are no PEP8 issues in the file pandas/tests/groupby/test_groupby.py !

Comment last updated on November 02, 2018 at 19:00 Hours UTC

[DiegoAlbertoTorres]

Ugh, had forgotten to amend my commit. Pep8 bot should be happy now.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ce8e05d). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master   #23463   +/-   ##
=========================================
  Coverage          ?   92.25%           
=========================================
  Files             ?      161           
  Lines             ?    51171           
  Branches          ?        0           
=========================================
  Hits              ?    47209           
  Misses            ?     3962           
  Partials          ?        0

Flag	Coverage Δ
#multiple	90.64% <100%> (?)
#single	42.28% <0%> (?)

Impacted Files	Coverage Δ
pandas/core/groupby/generic.py	87.07% <100%> (ø)

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[jreback]

@DiegoAlbertoTorres merge master and push again

[DiegoAlbertoTorres]

Done (rebased).

[jreback]

lgtm. ping on green.

[jreback]

thanks @DiegoAlbertoTorres