BUG: fix groupby.transform rename bug (#23461)
- [x] closes #23461
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @DiegoAlbertoTorres! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/groupby/generic.py`](https://github.com/DiegoAlbertoTorres/pandas/blob/2aa4dc0430e60e6b190959511dd16c5f1fa8c398/pandas/core/groupby/generic.py) !

 - There are no PEP8 issues in the file [`pandas/tests/groupby/test_groupby.py`](https://github.com/DiegoAlbertoTorres/pandas/blob/2aa4dc0430e60e6b190959511dd16c5f1fa8c398/pandas/tests/groupby/test_groupby.py) !



##### Comment last updated on November 02, 2018 at 19:00 Hours UTC Ugh, had forgotten to amend my commit. Pep8 bot should be happy now. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23463?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@ce8e05d`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23463/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23463?src=pr&el=tree)

```diff
@@            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](https://codecov.io/gh/pandas-dev/pandas/pull/23463?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/23463/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `87.07% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23463?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23463?src=pr&el=footer). Last update [ce8e05d...5f76e40](https://codecov.io/gh/pandas-dev/pandas/pull/23463?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @DiegoAlbertoTorres merge master and push again Done (rebased). lgtm. ping on green. thanks @DiegoAlbertoTorres 