Add task_id to Tasks/Get whitelist

Author: polyfractal

src/Elasticsearch/Endpoints/Tasks/Get.php

@@ -60,7 +60,8 @@ protected function getParamWhitelist()
             'parent_node',
             'parent_task',
             'wait_for_completion',
-            'group_by'
+            'group_by',
+            'task_id'
         );
     }