Currently, ActiveSupport::TimeWithZone#utc? simply runs a check to see
if the linked ActiveSupport::TimeZone's name is "UTC". This will only
return true for ActiveSupport::TimeZone["UTC"], but not for time zones
such as "Etc/UTC", "Etc/Universal", or other time zones that are aliases
for UTC. Interestingly enough, ActiveSupport::TimeWithZone#utc? is also
aliased as #gmt? but will return false for the "GMT" timezone (along
with other TZInfo aliases for GMT).

Instead of running a simple check on the TimeZone name, we can rely on
the underlying TZInfo::TimezonePeriod and TZInfo::TimezoneOffset which
keep a record of of the offset's abbreviated name. The possibilities
here for UTC time zones are `:UTC`, `:UCT`, and `:GMT`.

Signed-off-by: David <me@davidcel.is>