Previously, julian dates would not round trip through to_time.to_date,
because Time is always considered gregorian.  This converts the Date
instance from julian to gregorian before converting to Time, ensuring
that an equal date object will be returned if converting that Time
back to Date.

This does result in julian Date objects showing different day values
if converting to Time.

Fixes Ruby Bug 8428.

https://github.com/ruby/date/commit/d8df64555e