Set the pk of User object for Django 1.5 compatibility.

Setting the pk attribute of the User object for Django 1.5 compatibility.

In Django 1.4, the id of the User object was used as the unique identifier. In Django 1.5, the pk is now used instead of id.

Here is the related code change in Django: https://github.com/django/django/commit/0eeae15056edf07f786d3be5b47c14ab62eacd31