/**
  * Send asynchronous guzzle request
  *
  * @param Psr7Request $request
  * @param \Tebru\Retrofit\Http\Callback $callback
  * @return null
  */
 public function sendAsync(Psr7Request $request, Callback $callback)
 {
     $request = new Request($request->getMethod(), (string) $request->getUri(), $request->getHeaders(), $request->getBody(), ['future' => true]);
     /** @var FutureInterface $response */
     $response = $this->client->send($request);
     $this->promises[] = $response->then(function (ResponseInterface $response) {
         return new Psr7Response($response->getStatusCode(), $response->getHeaders(), $response->getBody(), $response->getProtocolVersion(), $response->getReasonPhrase());
     })->then($callback->success(), $callback->failure());
 }