Fix stack fault with dateConverter and with_current_user_as

Because of our simplifying of attr, we ended up being slightly more vulnerable to infinite loops of change-object-trigger-event-repeat.  This showed up in the date converter in Cacheable always returning a new Date object (which is never equal to the old one because of JS equality rules), which would always trigger change on the object that had the date as a property, which made with_current_user_as get triggered repeatedly (because it modelized the current user object from GGRC).  This fixes it so that if the new value of the date would be the same date, the old one is returned.  Boom, cycle broken.