UTC format timestamps in headers (#404) (#406)

This change makes all header timestamps (e.g. "Received") to use
UTC time zone.

This way the length of such headers do not depend on local time zone
which if effect fixes #404.

Signed-off-by: Saulius Gurklys <s4uliu5@gmail.com>

pkg/server/smtp/handler.go

@@ -533,7 +533,7 @@ func (s *Session) dataHandler() {
 	mailData := bytes.NewBuffer(msgBuf)
 
 	// Mail data complete.
-	tstamp := time.Now().Format(timeStampFormat)
+	tstamp := time.Now().UTC().Format(timeStampFormat)
 	for _, recip := range s.recipients {
 		if recip.ShouldStore() {
 			// Generate Received header.