Support field_collections in mongodb_migrate

Since #1653202: Initial field migration to MongoDB is closed, I'm moving MantasK's patch and problem here.

...btw with field collections it also was not working - host entity was not loaded. So I made some small change:

         if ('field_collection_item' == $entity_type) {
            //don't save host entity
            $entity->save(true);
          }
          else {
            entity_save($entity_type, $entity);
          }