QPID-3981: Windows C++ Broker fails ACL self tests

Most failures caused by a bug in NullAuthenticator, it was not adding the
default realm to user IDs that didn't already have a realm. 

Copied from trunk r1548268

git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.26@1548313 13f79535-47bb-0310-9956-ffa450edef68