Makes the 'created' and 'modified' keys optional: if $mongoSchema is defined without these keys

Thanks @YOMorales
#31

Author: ichikaway

models/datasources/mongodb_source.php

@@ -395,7 +395,7 @@ public function describe(&$Model, $field = null) {
 		$schema = array();
 		if (!empty($Model->mongoSchema) && is_array($Model->mongoSchema)) {
 			$schema = $Model->mongoSchema;
-			return $schema + $this->_defaultSchema;
+			return $schema + array('_id' => $this->_defaultSchema['_id']);
 		} elseif ($this->isConnected() && is_a($Model, 'Model') && !empty($Model->Behaviors)) {
 			$Model->Behaviors->attach('Mongodb.Schemaless');
 			if (!$Model->data) {