Handle scoped proxy properly in MBeanExporter

Previously, if a bean has a scoped proxy and is annotated to be exposed
to the JMX domain, both the scoped proxy and the target instance were
exposed in the JMX domain, resulting in a duplicate entries. Worse, if
such bean defines an explicit name, the application wouldn't start
because of a name conflict.

This commit deals explicitely with scoped proxy and make sure to only
expose the relevant bean.

Issue: SPR-12529