1139703 - update pickled schedule on schedule updates

Previously, the pickled CelerySchedule was being set on create but not update.

This patch checks to see if `iso_schedule` is being updated, and pickles an
updated CelerySchedule object to match the new `iso_schedule`.
Merge pull request #1170 from beav/pickled-schedule

1139703 - update pickled schedule on schedule updates
Merge remote-tracking branch 'origin/2.5-dev'