public function createHeader() {
    if ($this->router->isTestRun()) {
      return;
    }

    http_response_code($this->getStatusCode());
    if ($this->response === null) {
      // exception was raised
      // FIXME ensure that a response is always given initialized with at least the content type header
      header(Header::CONTENT_TYPE . ":" . $this->getContentType());
    } else {
      foreach ($this->response->getHeaders() as $header => $value) {
        header($header . ":" . $value);
      }
    }
  }