Values of fields attached to the forum taxonomy are empty after saving term through the forum interface.

Steps to reproduce:
- Enable forum module
- Add a field to the form taxonomy, for instance an image
- Add a forum term (admin/structure/forum)
- Edit this term, and add a value for the image, save.
(taxonomy/term/[tid])
- Edit the title of the term through the form term edit page, and save.
(admin/structure/forum/edit/container/[tid])

Now, the contents of the image field is gone.

A possible solutions might be:
- Merging the values on the existing term in forum_form_submit()
- Using the proper entity edit form in the edit page.