more FP avoidance tuning

git-svn-id: https://svn.apache.org/repos/asf/spamassassin/trunk@1623905 13f79535-47bb-0310-9956-ffa450edef68
more tuning, new test rule

git-svn-id: https://svn.apache.org/repos/asf/spamassassin/trunk@1623906 13f79535-47bb-0310-9956-ffa450edef68