Fix message rejection search bug

Author: mheap

src/Message/Client.php

@@ -217,7 +217,7 @@ public function searchRejections(Query $query) {
 
         $params = $query->getParams();
         $request = new Request(
-            \Nexmo\Client::BASE_REST . '/search/messages?' . http_build_query($params),
+            \Nexmo\Client::BASE_REST . '/search/rejections?' . http_build_query($params),
             'GET',
             'php://temp',
             ['Accept' => 'application/json']