Fix get_db_prep_save on JSONField for django 1.2

When you try to save a JSONField on newer djangos you would get:
* get_db_prep_save() takes exactly 3 arguments (2 given)

This fixes it by passing on the connection object (if specified) to
get_db_prep_save()
Merge pull request #6 from Tivix/fix_jsonfield

Fix get_db_prep_save on JSONField for django 1.2

https://docs.djangoproject.com/en/dev/howto/custom-model-fields/#converting-query-values-to-database-values