[TST] : Adding test cases to confirm that .apply() works fine
Adding test cases for closure of issue  :  Unexpected behaviour of apply function 

- [x] closes #21535
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@gfyoung  Please see if this is suffice for test case @uds5501 : Good start!  Can you include all of the examples from that issue under your test? Hello @uds5501! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on June 21, 2018 at 07:42 Hours UTC @gfyoung Thanks for pointing that out! That was really stupid by my side