feature #1001 Allow costume accept headers for GraphQL Endpoint. (Necmttn)

This PR was squashed before being merged into the 3.2.x-dev branch.

Discussion
----------

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

Commits
-------

6362281 Allow costume accept headers for GraphQL Endpoint.
dcd725e Update graphql.md
902e9ad Update lib/Github/Api/GraphQL.php
081953c Update lib/Github/Api/GraphQL.php

Author: Necmttn

doc/graphql.md

@@ -19,6 +19,19 @@ $client->authenticate($token, null, Github\Client::AUTH_ACCESS_TOKEN);
 $result = $client->api('graphql')->execute($query);
 ```
 
+#### Use different `Accept` Headers
+You can preview upcoming features and changes to the GitHub GraphQL schema before they are added to the GitHub GraphQL API.
+To access a schema preview, you'll need to provide a custom media type in the Accept header for your requests. Feature documentation for each preview specifies which custom media type to provide. More info about [Schema Previews](https://docs.github.com/en/graphql/overview/schema-previews).
+
+To use [GitHub v4 API (GraphQL API)](http://developer.github.com/v4/) with different `Accept` header you can pass third argument to execute method.
+
+```php
+$result = $client->api('graphql')->execute($query, [], 'application/vnd.github.starfox-preview+json')
+```
+> default accept header is `application/vnd.github.v4+json`  
+
+
+
 #### Use variables
 
 [Variables](https://developer.github.com/v4/guides/forming-calls/#working-with-variables) allow specifying of requested data without dynamical change of a query on a client side.

lib/Github/Api/GraphQL.php

@@ -18,12 +18,13 @@ class GraphQL extends AbstractApi
     /**
      * @param string $query
      * @param array  $variables
+     * @param string $acceptHeaderValue
      *
      * @return array
      */
-    public function execute($query, array $variables = [])
+    public function execute($query, array $variables = [], string $acceptHeaderValue = 'application/vnd.github.v4+json')
     {
-        $this->acceptHeaderValue = 'application/vnd.github.v4+json';
+        $this->acceptHeaderValue = $acceptHeaderValue;
         $params = [
             'query' => $query,
         ];