django-polymorphic does not work with django-parler

This fixes an issue when combining **django-polymorphic** with [django-parler](https://github.com/edoburu/django-parler).

This was only a problem, when adding a new object; editing existing objects did not cause this issue.

Here the context for rendering the form does not contain a ``form_url`` and thus the form's HTML tag ``action`` remains empty. Up to now this is not a problem, because then the form uses the current active URL on submission. However, when combining django-polymorphic with a similar module, say django-parler, the ``form_url`` gets overridden by ``?language=XX``, and therefore polymorphic does not “see” the GET parameter ``ct_id`` anymore and thus does not return the ``real_admin``.

References:
https://github.com/edoburu/django-parler/issues/25
https://code.djangoproject.com/ticket/21795