Fixed login issue with Django 1.7

The fiber_login view returns a HttpResponse with mimetype parameter. This is deprecated since Django 1.5 and has been effective in Django 1.7:
- https://docs.djangoproject.com/en/1.5/ref/request-response/#django.http.HttpResponse.__init__
- https://docs.djangoproject.com/en/1.7/ref/request-response/#django.http.HttpResponse.__init__