set method to null if missing in $_SERVER

fixes #18

src/Models/Partials/Request.php

@@ -13,7 +13,7 @@ class Request extends Arrayable
      */
     public function __construct()
     {
-        $this->method = $_SERVER['REQUEST_METHOD'];
+        $this->method = $_SERVER['REQUEST_METHOD']??null;
 
         $this->version = isset($_SERVER['SERVER_PROTOCOL'])
             ? \substr($_SERVER['SERVER_PROTOCOL'], \strpos($_SERVER['SERVER_PROTOCOL'], '/'))