Patch to fix authentication support for Django 1.5 and later.

When running MongoEngine as the database backend for Django 1.5 
and later, the django.contrib.auth.login and 
django.contrib.auth.authenticate methods fail here, since the 
`update_fields` keyword argument is not present in the `save()` 
method. This error is caused by Django [here](https://github.com/django/django/blob/master/django/contrib/auth/models.py#L244)
in the `check_password` method of the User object.