TST: Same return values in drop_duplicates for Series and DataFrames(#…
 - [x] closes #14192
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

…14192)
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14754?src=pr) is 85.24% (diff: 100%)


> No coverage report found for **master** at 725453d.

> Powered by [Codecov](https://codecov.io?src=pr). Last update [725453d...8b84d12](https://codecov.io/gh/pandas-dev/pandas/compare/725453deb2efae1b84d87131e785a42a22753659...8b84d128ebeb092e7027753f49d9c8622c3ea0d6?src=pr) Added the `keep` kwarg and added `int` and `datetime64` columns @sinhrks  @mroeschke Thanks!