Add another RFC data based test

dkim/tests/data/rfc6376.signed.msg

@@ -0,0 +1,19 @@
+DKIM-Signature: v=1; a=ed25519-sha256; c=simple/simple; 
+ d=football.example.com; [email protected]; 
+ q=dns/txt; s=brisbane; t=1518460054; h=from : to : 
+ subject : date : message-id : from : subject : date; 
+ bh=4bLNXImK9drULnmePzZNEBleUanJCX5PIsDIFoH4KTQ=; 
+ b=9/dsDChY0YMTtD5Eyw3wx7x22BlSJP7M5ECbJ7GWrR45nXlTCGb8l0YB
+ o0wBLR++X5LqmsxXaOYLLJe46l10AQ==
+From: Joe SixPack <[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?
+
+Joe.
+