private function request(array $params = array())
  {
    $request = $this->client->createRequest('POST', $this->url);

    $body = $request->getBody();
    foreach ($params as $key => $value) {
      $body->setField($key, $value);
    }

    try {
      $response = $this->client->send($request);
    } catch (\Exception $e) {
      throw new \ScottRobertson\Premailer\Exception\Request($e->getMessage());
    }

    return new Response(
      $response->json(),
      $this->client
    );
  }