Fixed GitHub response parsing

src/Appwrite/Auth/OAuth2/Github.php

@@ -53,7 +53,7 @@ class Github extends OAuth2
     protected function getTokens(string $code): array
     {
         if(empty($this->tokens)) {
-            $this->tokens = \json_decode($this->request(
+            $response = $this->request(
                 'POST',
                 'https://github.com/login/oauth/access_token',
                 [],
@@ -63,7 +63,11 @@ class Github extends OAuth2
                     'client_secret' => $this->appSecret,
                     'code' => $code
                 ])
-            ), true);
+            );
+
+            $output = [];
+            \parse_str($response, $output);
+            $this->tokens = $output;
         }
 
         return $this->tokens;