Fix @SubscribeMapping MESSAGE response destination

Prior to this commit, @SubscribeMapping mapped methods (backed with
@SendTo* annotations, or not) would send MESSAGEs with the wrong
destination. Instead of using the original SUBSCRIBE destination, it
would use the lookup path computed from the configured prefixes in the
application.

This commit fixes this issue - now @SubscribeMapping MESSAGEs use the
original SUBSCRIBE destination.

Issue: SPR-11648