Set correct generic type parameters in CombinedConfigurationBuilder.

A ClassCastException was thrown in the tests because results of
configurationsAt() were cast to SubnodeConfigurations. The definition
configuration can now have an arbitrary node type.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1588687 13f79535-47bb-0310-9956-ffa450edef68