BUG: Allow TZ-aware DatetimeIndex in merge_asof() (#14844)
 - [x] closes #14844 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry


lgtm. ping on green. @jreback So Travis passed but AppVeyor failed because of the one-hour time out. Should I just resubmit? I restarted the job, but very odd have seen that a lot recently. And some succeed just fine. thanks!