Plugin/DKIM.pm: adapt to a last-minute change in Mail::DKIM before its 0.40 release, [rt.cpan.org #83170], [rt.cpan.org #80425]

git-svn-id: https://svn.apache.org/repos/asf/spamassassin/trunk@1443294 13f79535-47bb-0310-9956-ffa450edef68