Add `with_primary_key` to AssociationMatchers

This adds `with_primary_key` to `AssociationMatchers` so you can now test the `:primary_key` option of an association.
It's not so much that the class has a primary key... it's that the primary key option is correct (i.e. matches the one on the association). Do you mind renaming this method to match the other methods that check options? (i.e. `primary_key_correct?`)