Fix issue in GlassfishRequestUpgradeStrategy

The observed behavior was that the client does not get a response from
the WebSocket HTTP handshake. On the server the handshake actually
succeeds, the response is set correctly to status 101, and the
WebSocketHandler gets notified of the successfully established
connection.

This change flushes the ServletResponse just before returning from the
GlassfishRequestUpgradeStrategy. This is actually what Glassfish's own
TyrusServletFilter does as well at the end along with a comment that it
is a possible bug.
Only consider "is" methods with boolean returns

Fix regression introduced in b25e91a5 where ReflectivePropertyAccessor
does not consider the return type for "is" getters.

Issue: SPR-11142
Release version 4.0.0.RC2
Increment version to 4.0.0.BUILD-SNAPSHOT
Document ControllerAdvice annotation

Issue: SPR-11149
Polishing
Delete sysout.println call from MBean*Tests
Delete unused import in MockAsyncContext
Update Javadoc, add tests and polish spring-messaging