Support XML properties in ResourcePropertySource

JDK 5 introduced an XML-based properties file syntax. This commit
ensures that when such files are supplied as the underlying resource
for a ResourcePropertySource instance, they are routed appropriately
to Properties#loadFromXML as opposed to Properties#load.

Issue: SPR-9896
Merge branch 'SPR-9896' into 3.2.x

* SPR-9896:
  Support XML properties in ResourcePropertySource