Fix loading PostgreSQL primary keys

It has been broken for quite a long time when somebody made a change to this file (not original developer), so I have finally decided to fix it.
Without my patch, this method will return unique constraints and foreign keys along with actual primary key as if they all were primary keys, which is obviously not true. Then certain commands will fail, as they rely on primary keys (e.g. UPDATE). It's simply a bad join.
After the change, only actual primary key constraints are returned. Tested with PostgreSQL 9.1.