Fixed a problem with BaseHierarchicalConfiguration.clone() related to SubnodeConfiguration management.

It must be ensured that the cloned configuration creates its own data
structures for managing SubnodeConfigurations.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1488572 13f79535-47bb-0310-9956-ffa450edef68