@Override
        public MessageBusVisitorSession.Receiver createReceiver(MessageHandler messageHandler,
                                                                 String sessionName) {
            MockReceiver ret = receiver;
            if (ret == null) {
                throw new IllegalStateException("Attempted to create mock receiver twice");
            }
            ret.setMessageHandler(messageHandler);
            receiver = null;
            return ret;
        }
Invoke registered MessageHandler with message
@param message message to "send"
documentapi/src/test/java/com/yahoo/documentapi/messagebus/MessageBusVisitorSessionTestCase.java