ENH: try to preserve the dtype on combine_first for the case where the two DataFrame objects have the same columns
…e two DataFrame objects have the same columns

- [ ] closes #7509 
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Hello @danielhrisca! 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 2021-01-15 14:44:28 UTC @jreback I've changed the implementation and added tests I think all request have been met. The failed test does not seem to be related to the changes done in this PR, most probably come after I re-synced with the master branch  @jreback I've done the changes  but again there are failed builds for which I can see no connection with this PR minor nitpick, otherwise looks good thanks @danielhrisca very nice