local_to_utc should return UTC when system time is UTC

[hamiltop]

Because of the way Ruby handles timezones we can find ourselves in a position where a Time object is non-utc but has a timezone of UTC. If the system timezone utc, we get the following

irb(main):013:0> Time.now.utc?
=> false
irb(main):014:0> Time.now.zone
=> "UTC"

If we use such a time object in calling local_to_utc, the is a bug in https://github.com/tzinfo/tzinfo/blob/master/lib/tzinfo/time_or_datetime.rb#L52

In that line we check for UTC in the zone. I propose we check the utc? method instead of checking the zone method.

The result is that local_to_utc may return a non-utc Time object with zone UTC. With the proposed fix we should always be returning a utc Time object.

I'm happy to submit the fix. Just wanted some feedback first.

[philr]

Thank you for the bug report.

I have committed a fix (changed to use utc? instead of zone as you propose).