getSection() now returns a SubnodeConfiguration.

Added some tests which ensure that the sub configurations are actually
connected to their parent.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1580608 13f79535-47bb-0310-9956-ffa450edef68
Fixed a failing test case.

The semantics of detached sub configurations has slightly changed. Therefore,
the test needs to be adapted: By calling close() the tracked node gets freed,
and a new section sub configuration can be created which is again connected to
its parent.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1580609 13f79535-47bb-0310-9956-ffa450edef68