Reworked CombinedConfiguration to operate on immutable nodes.

The majority of tests is working, but there are still some test failures.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1587015 13f79535-47bb-0310-9956-ffa450edef68
Fixed test case for the copy constructor.

It is necessary to invoke beginRead() on the configuration to be copied.
Otherwise, the combined configuration does not get constructed.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1587016 13f79535-47bb-0310-9956-ffa450edef68