private void sendStringMessages() {
        Message<?> message = new GenericMessage<String>("ping");
        for (int i = 0; i < numMessages; i++) {
            toStringHandlerChannel.send(message);
        }
    }
Test the RxJava based message handlers by using two types of {@link Processor}. The first is parameterized by
{@link org.springframework.messaging.Message} and the second by String to test extracting payload types and
wrapping return types in a Message.

@author Mark Pollack
spring-xd-rxjava/src/test/java/org/springframework/xd/rxjava/AbstractMessageHandlerTests.java