[3.0.x backport] 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`.

Conflicts:
	spring-integration-core/src/main/java/org/springframework/integration/config/AbstractSimpleMessageHandlerFactoryBean.java

Resolved.

INT-3565 Polishing

- Fix test.
- enhance test to show that directly bound message handler beans are not advised if already proxied.

Conflicts:
	spring-integration-core/src/main/java/org/springframework/integration/config/AbstractSimpleMessageHandlerFactoryBean.java

Resolved.