docs: public static void prepareUpgrade(final ServerEndpointConfig config, final WebSocketHttpExchange exchange) {
          ExchangeHandshakeRequest request = new ExchangeHandshakeRequest(exchange);
          ExchangeHandshakeResponse response = new ExchangeHandshakeResponse(exchange);
          ServerEndpointConfig.Configurator c = config.getConfigurator();
          c.modifyHandshake(config, request, response);
          response.update();
      }