Added a new mergeRoot() method to InMemoryNodeModel.

Such a merge operation is required by concrete configuration implementations
when a new configuration source has been loaded whose data now has to be
added to the already existing content.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1585193 13f79535-47bb-0310-9956-ffa450edef68