Add: grantType to server based on indentifier

If $key is not provided Server::addGrantType is called, add the grant type base on qyery string identifier for better consistency

src/OAuth2/Server.php

@@ -350,7 +350,7 @@ public function addGrantType(GrantTypeInterface $grantType, $key = null)
         if (is_string($key)) {
             $this->grantTypes[$key] = $grantType;
         } else {
-            $this->grantTypes[] = $grantType;
+            $this->grantTypes[$grantType->getQuerystringIdentifier()] = $grantType;
         }
 
         // persist added grant type down to TokenController