Reverted addJoinObject methods to old behaviour.

This was added in Propel's #641 pull request. It breaks multiple column joins, as per my comments on that pull request.

Quick demo:

$q = ExampleModelQuery::create();

$q->addJoin(array(ExampleModelPeer::COLUMN_A, ExampleModelPeer::COLUMN_B), array(RelatedModelPeer::COLUMN_A, RelatedModelPeer::COLUMN_B), Criteria::INNER_JOIN);

var_dump(count($q->getJoins()));

> int(1)

$q->addJoin(array(RelatedModelPeer:COLUMN_C, RelatedModelPeer::COLUMN_D), array(AnotherRelatedModelPeer::COLUMN_A, AnotherRelatedModelPeer::COLUMN_B), Criteria::INNER_JOIN);

var_dump(count($q->getJoins()));

> int(1)

The second join doesn't get added.