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`.
So, does this test show that our `handler` in the `createHandlerInternal()` above is really `Proxy` ?
Just make an answer and I'll merge it