Added simple tests for the assign to dossier functionality.

Which tests both use cases assign to a new and to existing dossier.
Fixed url generation in assign forwarding to a new dossier functionality.

Actually the code has raised the following attribute Error:
```
Module opengever.task.browser.assign_dossier, line 103, in handle_continue
AttributeError: 'function' object has no attribute 'admin_unit'
```