INT-3565 ClassCastException with Proxy and DEBUG

JIRA: https://jira.spring.io/browse/INT-3565

Cast to `IntegrationObjectSupport` fails if the handler is
already proxied in `AbstractSimpleMessageHandlerFactoryBean`.

INT-3565 Polishing

- Fix test.
- enhance test to show that directly bound message handler beans are not advised if already proxied.

spring-integration-core/src/test/java/org/springframework/integration/transformer/TransformerContextTests.java

@@ -31,6 +31,8 @@ import org.springframework.messaging.PollableChannel;
 import org.springframework.messaging.support.GenericMessage;
 
 /**
+ * Also in JMX - changes here should be reflected there.
+ *
  * @author Mark Fisher
  * @author Gary Russell
  */