Fix for *old* way of storing tags. B/C issue in J!3.1.2.

This will fix the *old* way of storing tags for 3rd party extensions and it will not do anything for those who store it the new way.

This commit is based on the solution Michael Babker used for his extension (see https://github.com/mbabker/Podcast-Manager/blob/master/com_podcastmanager/admin/models/podcast.php#L631), but taken to the legacy form model and adjusted so it doesn't interfere with extensions that don't need it.

Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31598