Don't cast pk to int (again).

This commit -- http://github.com/jezdez/celery-haystack/commit/39edd7f86fc5c0ddf69d73ebe5114163b682ddfb -- was overwritten by a bad merge here -- http://github.com/jezdez/celery-haystack/commit/4c434bf78c5ca3db9e7d813d1d3d826cfc609d03.