Hook to support STI subclasses of Tag in save_tags

One use case I have is that a custom Tag class is "followable" (see way back to #218). Because `Core#save_tags` is a monstrous method, I was hacking around it on an override using `with_scope` around `super` -- this eliminates the need for such a dirty trick.

This is possibly incomplete support for `Tag` being robustly subclass-able, but I haven't run into any other issues yet with the features of acts-as-taggable-on that we're using.