Fixed url generation in assign forwarding to a new dossier functionality.

The actual code failed with the following `AttributeError`, when assigning a forwarding to a new dossier:
```
Traceback (innermost last):
  Module ZPublisher.Publish, line 60, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 46, in call_object
  Module plone.z3cform.layout, line 70, in __call__
  Module plone.z3cform.layout, line 54, in update
  Module z3c.form.form, line 210, in update
  Module z3c.form.action, line 99, in execute
  Module z3c.form.button, line 315, in __call__
  Module z3c.form.button, line 170, in __call__
  Module opengever.task.browser.assign_dossier, line 103, in handle_continue
AttributeError: 'function' object has no attribute 'admin_unit'
```

Additionally i've added two simple tests, which tests both use cases assign to a new and to existing dossier.

@deiferni please have a look ...