Destroyed class loader should still delegate to its parent.

A unified class loader that has been destroyed as part of an undeployment should try to delegate to its parent in this state. If it does not, even trying to load java system classes can fail. An example of where this has shown up is a mbean service trying to create a thread in its stop method ( the org.jboss.test.jmx.shutdown.ExitOnShutdown service used by the minimal server test).