Improve ActiveSupport::TimeWithZone conversion to YAML

Previously when converting AS::TimeWithZone to YAML it would be output
as a UTC timestamp. Whilst this preserves the time information accurately
it loses the timezone information. This commit changes that so that it is
saved along with the time information. It also provides nicer encoding of
AS::TimeZone instances themselves which previously embedded all of the
data from the TZInfo records.

Fixes #9183.