Allow tz-aware inputs in Holiday.dates

[ssanderson]

closes #9825

Previously, passing tz-aware inputs to Holiday.dates would always result in an error because the the input start_date was converted in a way that destroyed timezone information. This PR makes that conversion correctly preserve tz info, and converts several of the existing tests to run with both tz-aware and tz-naive inputs.

Noticed while working on a new trading calendar implementation here: quantopian/zipline#556.

[jreback]

your tests don't check for a holiday with a timezone. wasn't this the use case?

[ssanderson]

@jreback the specific issue solved here is that calling holiday.dates(start, end) with tz-aware start and end raises an error because dates attempts to construct a DatetimeIndex with a tz-naive start and the supplied end. The fix here just propagates the timezone of the supplied start so that the call succeeds if both inputs are tz-aware and in the same timezone. That's tested at https://github.com/pydata/pandas/pull/9824/files#diff-96b913a0de65ff4f70fc5bde0543e9a8R62 by running the same checks that were performed previously, but with dates that are localized to UTC. The substantive content of the testing changes are in ssanderson@8ae03df. The rest of the diff is just refactoring to make it easier to run the same check twice, once with tz-naive inputs, and once with tz-aware inputs.

[jreback]

@ssanderson ok, I see, then ok!

pls squash and good to go

[ssanderson]

@jreback squashed into one commit and rebased with master. I assume that's what you meant?

[jreback]

@ssanderson yep

thanks for the PR!

[ssanderson]

@jreback great! #9825 can be closed (I mistyped in my issue description).

[jreback]

fixed thanks!