fix and test for: https://issues.apache.org/jira/browse/AMQ-4413

For durable subscriptions the PrefetchSubscription can't drop messages in dispatchPending otherwise they are never added back into the pending cursor. 

git-svn-id: https://svn.apache.org/repos/asf/activemq/trunk@1483111 13f79535-47bb-0310-9956-ffa450edef68