Add unittest for checkDKIM() with `X-DKIM-Authentication-Results: dunno`.

 * ADD new unittest test_checkDKIM_dunno() to test_email_dkim.py which checks
   that a ``X-DKIM-Authentication-Results: dunno`` header properly returns
   False.

lib/bridgedb/test/test_email_dkim.py

@@ -62,3 +62,14 @@ get bridges
         result = dkim.checkDKIM(message,
                                 self.domainRules.get("gmail.com"))
         self.assertIs(result, False)
+
+    def test_checkDKIM_dunno(self):
+        """A ``X-DKIM-Authentication-Results: dunno`` header should return
+        False.
+        """
+        messageList = self.badMessage.split('\n')
+        messageList[2] = "X-DKIM-Authentication-Results: dunno"
+        message = self._createMessage('\n'.join(messageList))
+        result = dkim.checkDKIM(message,
+                                self.domainRules.get("gmail.com"))
+        self.assertIs(result, False)