code: protected boolean sendSingleLogoutMessage(final SingleLogoutRequest request, final SingleLogoutMessage logoutMessage) {
          val logoutService = request.getService();
          LOGGER.trace("Preparing logout request for [{}] to [{}]", logoutService.getId(), request.getLogoutUrl());
          val msg = getLogoutHttpMessageToSend(request, logoutMessage);
          LOGGER.debug("Prepared logout message to send is [{}]. Sending...", msg);
          val result = sendMessageToEndpoint(msg, request, logoutMessage);
          logoutService.setLoggedOutAlready(result);
          return result;
      }
docs: Send single logout message.
  
  @param request       the request
  @param logoutMessage the logout request
  @return true if the message was successfully sent.