Int 3460: Support output-channel when an AbstractCorrelatingMessageHandler is 'ref'd by a Service Activator	

INT-3460: Support output-channel when an AbstractCorrelatingMessageHandler is 'ref'd by a Service Activator	

* allow AbstractCorrelatingMessageHandler's output referenced by service-activator output-channel
* add test 
@garyrussell , are you fine with this name? 
How about just `MessageProducingHandler` ?