mail/spamassassin: update to 3.4.6 release (+)

Apache SpamAssassin 3.4.6 fixes two bugs that were introduced by the
3.4.5 release that could prevent certain rules from properly firing.

Changelog: https://svn.apache.org/repos/asf/spamassassin/branches/3.4/build/announcements/3.4.6.txt