this  field shouldn't be serialized but it can since it is in AppInfo through PersistenceUnitInfo#properties so ensureing we don't break everything

git-svn-id: https://svn.apache.org/repos/asf/tomee/tomee/branches/tomee-1.7.x@1631093 13f79535-47bb-0310-9956-ffa450edef68