Hook to support STI subclasses of Tag in save_tags

One use case I have is that a custom Tag class is "followable". Because
`Core#save_tags` is a monstrous method, I was dirtily hacking around it
on an override using `with_scope` around `super` -- this eliminates the
need for such a trick.
Apply Tag STI hook for owned taggings
Merge pull request #413 from ches/hook-for-tag-sti-subclass

Hook to support STI subclasses of Tag in save_tags