InMemoryNodeModel.setProperty() now handles value changes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1571055 13f79535-47bb-0310-9956-ffa450edef68
InMemoryNodeModel.setProperty() now can deal with empty operations.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1571056 13f79535-47bb-0310-9956-ffa450edef68