OAK-1951 : UserContext doesn't reflect new group-member structure

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1607849 13f79535-47bb-0310-9956-ffa450edef68
OAK-1943 : additional test provided by dominique jaeggi (thanks!)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1607857 13f79535-47bb-0310-9956-ffa450edef68