Symfony\Component\HttpFoundation\Response::setCharset PHP Method

    public function setCharset($charset)
    {
        $this->charset = $charset;

        return $this;
    }

 protected function sendResponse($payload = NULL)
 {
     $this->res->setExpiration(FALSE);
     if (is_null($payload)) {
         $this->res->setStatusCode(204);
     } else {
         $this->res->setCharset('utf8');
         $this->res->headers->set('Content-Type', 'application/json');
         $content = json_encode($payload, JSON_PRETTY_PRINT);
         $this->res->setContent($content);
     }
     $this->res->send();
 }