Added another test for getSource() with a nested combined configuration.

Changed a test with sub configurations to expose a problem with updating a
combined configuration: When accessing the model of a combined configuration
that has been invalidated it is not always reconstructed.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1587021 13f79535-47bb-0310-9956-ffa450edef68