Added a clearProperty() method to InMemoryNodeModel.

The method is partly implemented for properties represented by nodes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1571048 13f79535-47bb-0310-9956-ffa450edef68
InMemoryNodeModel.clearProperty() now handles attributes correctly.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1571049 13f79535-47bb-0310-9956-ffa450edef68