BUG: infer_freq results in None for hourly freq with timezone

Found fix #7318 was incorrect. It works for lower frequencies than dates, but not for higher freq than hours.
 I couldn't detect as test cases are also incorrect...

This should work for all the freqs with timezones.

NOTE: I couldn't use normal `tslib.tz_convert` which returns incorrect results maybe caused by DST. Applying `tslib.tz_convert_single` can work though.