Add SmartCollection Metafield child resource

Author: tompec

lib/SmartCollection.php

@@ -36,6 +36,7 @@ class SmartCollection extends ShopifyResource
      */
     protected $childResource = array(
         'Event',
+        'Metafield',
     );
 
     /**
@@ -51,4 +52,4 @@ public function sortOrder($params)
 
         return $this->put(array(), $url);
     }
-}
+}