Set header to accept v4 API

Author: m1guelpf

lib/Github/Api/GraphQL.php

@@ -2,6 +2,7 @@
 
 namespace Github\Api;
 
+use AcceptHeaderTrait;
 /**
  * GraphQL API.
  *
@@ -12,13 +13,15 @@
  */
 class GraphQL extends AbstractApi
 {
+    use AcceptHeaderTrait;
     /**
      * @param string $query
      *
      * @return array
      */
     public function execute($query)
     {
+        $this->acceptHeader = 'application/vnd.github.v4+json'; 
         $params = array(
             'query' => $query
         );