Add 'stored_fields' param to Exists, Explain and Search endpoints

Closes #554

Author: polyfractal

src/Elasticsearch/Endpoints/Exists.php

@@ -60,6 +60,7 @@ public function getParamWhitelist()
             'refresh',
             'routing',
             'version',
+            'stored_fields'
         );
     }
 

src/Elasticsearch/Endpoints/Explain.php

@@ -86,6 +86,7 @@ public function getParamWhitelist()
             '_source',
             '_source_exclude',
             '_source_include',
+            'stored_fields'
         );
     }
 

src/Elasticsearch/Endpoints/Search.php

@@ -95,6 +95,7 @@ public function getParamWhitelist()
             'docvalue_fields',
             'filter_path',
             'terminate_after',
+            'stored_fields'
         );
     }