allow defining a default handler stack

src/Http/Senders/GuzzleSender.php

@@ -66,11 +66,7 @@ public function getFactoryCollection(): FactoryCollection
      */
     protected function createGuzzleClient(): GuzzleClient
     {
-        // We'll use HandlerStack::create as it will create a default
-        // handler stack with the default Guzzle middleware like
-        // http_errors, cookies etc.
-
-        $this->handlerStack = HandlerStack::create();
+        $this->handlerStack = $this->defaultHandlerStack();
 
         // Now we'll return new Guzzle client with some default request
         // options configured. We'll also define the handler stack we
@@ -86,6 +82,18 @@ protected function createGuzzleClient(): GuzzleClient
         ]);
     }
 
+    /**
+     * The default handler stack used by the underlying Guzzle client
+     */
+    protected function defaultHandlerStack(): HandlerStack
+    {
+        // We'll use HandlerStack::create as it will create a default
+        // handler stack with the default Guzzle middleware like
+        // http_errors, cookies etc.
+
+        return HandlerStack::create();
+    }
+
     /**
      * Send a synchronous request.
      *