in richtext (ck) editor, deleting one custom tag deletes more than one.

to reproduce in clean 4.5 install:

1. Create a youTube custom tag.
2. Create another custom tag, this time a twitter one. 
3. And then create a facebook one and another twitter one.
4. You should now have four on the page.
5. Try and delete the last twitter one. 
6. The twitter and the facebook one above are both deleted.
7. Hit control-z and see that the unintended facebook one comes back, and the twitter one stays deleted.