API/BUG: a UTC object inserted into a Series/DataFrame will preserve the UTC and be object dtype (GH8411)
closes #8411 

cc @rockg
cc @ischwabacher
cc @adamgreenhall

This actually wasn't very difficult and is not really a very big change IMHO (as rarely would you actually do this). Came up because couldn't chain ops like this:

`s.dt.tz_localize('UTC').dt.tz_convert('US/Eastern')`
