Allow costume accept headers for GraphQL Endpoint.

Github has multiple different accept headers for different purposes.  Currently, there's no easy way to add/remove the `Accept` header to the client. 

ref: https://docs.github.com/en/graphql/overview/schema-previews

Author: Necmttn

lib/Github/Api/GraphQL.php

@@ -21,9 +21,9 @@ class GraphQL extends AbstractApi
      *
      * @return array
      */
-    public function execute($query, array $variables = [])
+    public function execute($query, array $variables = [], $acceptHeaderValue = 'application/vnd.github.v4+json')
     {
-        $this->acceptHeaderValue = 'application/vnd.github.v4+json';
+        $this->acceptHeaderValue = $acceptHeaderValue;
         $params = [
             'query' => $query,
         ];