Add 'max_concurrent_shard_requests' param to MSearch endpoint

Author: polyfractal

src/Elasticsearch/Endpoints/Msearch.php

@@ -80,6 +80,7 @@ public function getParamWhitelist()
         return array(
             'search_type',
             'typed_keys',
+            'max_concurrent_shard_requests'
         );
     }