Added test for SpringProperties setFlag method

Issue: SPR-9014
Issue: SPR-11297
(cherry picked from commit 23249cd)
Drop Introspector.flushFromCaches calls completely

According to the JDK's documentation and changelog, the Introspector itself safely handles weak references as of JDK 6 update 21 (which is what we require for Spring 4.0).

Issue: SPR-11356
Polishing

Issue: SPR-11357