@Test
    public void stringBasedProcessor() throws IOException {

        sendStringMessages();
        for (int i = 0; i < numMessages; i++) {
            Message<?> outputMessage = outputChannel2.receive(500);
            assertEquals("ping-stringpong", outputMessage.getPayload());
        }
    }
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