Fix faulty placeholder in has many forms.

2133d7cdf6ec9d85406a95af9a6496306ef6492e introduced an error in all has_many forms with namespaced models.
Because ids cannot contain a forward-slash it will be rewritten by rails helpers to an underscore. So the javascript could not replace the placeholders anymore.
Added a test to capture future breaks
Merge pull request #3460 from DiMarcello/feature/namespaced-has_many

Fix faulty placeholder in has many forms.