Fixed some problems introduced in the latest merge operation.

Some versions had been overridden and had to be restored.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1585178 13f79535-47bb-0310-9956-ffa450edef68
Added XMLDocumentHelper class.

This class will be used by XMLConfiguration to store a loaded XML document.
It also provides functionality for copying documents which is needed by
thread-safe save operations.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1585179 13f79535-47bb-0310-9956-ffa450edef68