BUG: resample with tz-aware: Values falls after last bin
 - [ ] closes https://github.com/pandas-dev/pandas/issues/15549
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry: resampling will now always produce bins alligned by UTC instead of custom time zone. so for example if you resampling by 12 hours frequency in UTC-5 timezone, you will get bins like
7:00 19:00 7:00 19:00
instead of 
00:00 12:00 00:00 12:00

jreback, would you like me to add tests for this change ? tests!

*always*

and a whatsnew note please. committed a test and wrote a whatsnew note.
please let me know if the format of whatsnew is correct, I'm not sure if there is any standard form I should follow. I understand why the checks failed, I will commit a fix @jreback is it ok to change already existing tests in order to make my change pass them ? you need to prove that a current test is wrong in order to change it I think you need to compute the range edges *with* the tz intact. I think it is mishandling when it has an ambiguous datetime.

Then constructing the tz should be ok. got it, let me try to use your approach @ahcub yeah this seemingly trivial change is actually hard. there have been lots of fixes (for various reasons) thru the years. good news is the test suite is quite comprehensive here :> can you rebase / update I will close it for now and restore it when I get back to it @ahcub ok sure. though pls come back when you can! thanks!