InMemoryNodeModel.addNodes() now verifies the key pointing to the target node.

It the key references an attribute, an exception is thrown as it is not
possible to add nodes to an attribute.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1571061 13f79535-47bb-0310-9956-ffa450edef68