Do not force the database when one is explicitly specified.

It is EXTREMELY dangerous to do this. E.g.,

$client = new Horde_Mongo_Client();

// This inserts into 'horde' even though you specify 'test_database'.
$client->test_database->my_collection->insert($document);

// This completely nukes your 'horde' database.
$client->test_database->drop();