The `Query` object shouldn't overwrite custom values with model-supplied values.

Source

						public function testQueryWithCustomAlias() {
		$model = 'lithium\tests\mocks\data\model\MockQueryComment';

		$query = new Query(compact('model') + array(
			'source' => 'my_custom_table',
			'alias' => 'MyCustomAlias'
		));
		$result = $query->export(Connections::get('mock-database-connection'));
		$this->assertEqual('{my_custom_table}', $result['source']);
		$this->assertEqual('AS {MyCustomAlias}', $result['alias']);
	}