Add test test_non_utf8 which at least demonstrates not crashing on non UTF-8 data

dkim/tests/data/rfc6376.w1258.msg

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+From: Tomá? SixPack <Tomá[email protected]>
+To: Suzie Q <[email protected]>
+Subject: Is dinner ready?
+Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
+Message-ID: <[email protected]>
+
+Hi.
+
+We lost the game.  Are you hungry yet?
+
+Tomá?.
+