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`.
Yes, for some reason, the router is already proxied before getting here.

The offending log was added in 4.1 so that won't need a backport. Nobody has reported a problem with the advice chain not being applied to proxies; I need to do some more research before deciding whether we should backport that part.