BUG: Fix combine_first converts other columns type into floats unexpectedly
- [x] closes #20699
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20965?src=pr&el=h1) Report
> Merging [#20965](https://codecov.io/gh/pandas-dev/pandas/pull/20965?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/66c2e5fe5078262e6c99022b3a7404c2102045af?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20965/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20965?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20965      +/-   ##
==========================================
- Coverage   92.19%   92.19%   -0.01%     
==========================================
  Files         169      169              
  Lines       50873    50879       +6     
==========================================
+ Hits        46904    46908       +4     
- Misses       3969     3971       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.32% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20965?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20965/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.11% <100%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20965?src=pr&el=continue).
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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20965?src=pr&el=footer). Last update [66c2e5f...df89c71](https://codecov.io/gh/pandas-dev/pandas/pull/20965?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @jreback, fixed! can you add a whatsnew note. also rebase on master; I cleaned some older code out of here (shouldn't impact but just to be sure) @Licht-T Do you have time to update this? can you rebase / update?

@gfyoung if not, can you fix the note Hello @Licht-T! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/Licht-T/pandas/blob/361113ab727d56f1c40e5e9abd9d3752c23ddd14/pandas/core/frame.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_combine_concat.py`](https://github.com/Licht-T/pandas/blob/361113ab727d56f1c40e5e9abd9d3752c23ddd14/pandas/tests/frame/test_combine_concat.py) !

 @jreback : Comments addressed, and all is rebased and green. PTAL. @jreback : Comments addressed, and all is rebased and green. PTAL. thanks @Licht-T (and @gfyoung )!

note that this may close other combine / combine_first issues. maybe have a look if you have a chance.