PropertySourcePropertyValues tries to process a PropertySource first
as an EnumerablePropertySource and then as a CompositePropertySource.
In Spring 4.1.2 CompositePropertySource was updated to extend
EnumerablePropertySource. This change meant that a
CompositePropertySource would always be processed as an
EnumerablePropertySource.

This commit updates PropertySourcePropertyValues to process a
PropertySource as a CompositePropertySource first and to then try it
is an EnumerablePropertySource.

Fixes gh-2608