QPIDJMS-9: move responsibility for implementing clearBody() to the concrete message classes

git-svn-id: https://svn.apache.org/repos/asf/qpid/jms/trunk@1568702 13f79535-47bb-0310-9956-ffa450edef68
QPIDJMS-9: work on support for JMSXGroupID

git-svn-id: https://svn.apache.org/repos/asf/qpid/jms/trunk@1568703 13f79535-47bb-0310-9956-ffa450edef68