fix parsing Authentication-Results header with a "reason=..." part

noticed in gopherwatch logging

message/authresults.go

@@ -424,6 +424,8 @@ func (p *parser) xresinfo(methodKeyword string) (am AuthMethod) {
 	am.Method, am.Version, am.Result = p.xmethodspec(methodKeyword)
 	p.cfws()
 	if p.take("reason") {
+		p.cfws()
+		p.xtake("=")
 		p.cfws()
 		am.Reason = p.xvalue()
 	}

message/authresults_test.go

@@ -311,6 +311,22 @@ func TestAuthResultsParse(t *testing.T) {
 		},
 	})
 
+	const dkimReason = `host.example;
+ dkim=none reason="no dkim signatures"
+`
+	ar, err = ParseAuthResults(dkimReason)
+	tcheck(t, err, "parsing auth results header")
+	tcompare(t, ar, AuthResults{
+		Hostname: "host.example",
+		Methods: []AuthMethod{
+			{
+				Method: "dkim",
+				Result: "none",
+				Reason: "no dkim signatures",
+			},
+		},
+	})
+
 	// Outlook adds an invalid line, missing required hostname at the start. And their
 	// dmarc "action=none" is invalid. Nothing to be done.
 	const outlook = `x; spf=pass (sender IP is 84.22.96.237)