Backport "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
Backport-Commit: 3a626f93197d7f0fd4266d9877550a8d330017e5