fix bug with dkim signing messages without Date or Message-Id header

we were adding the missing date and/or message-id header, but didn't sign it.
and the default dkim signing config is to (over)sign those headers. so that was
causing errors with bad signatures.

found while setting up automated tests for quickstart, while sending a very
basic message between a fresh install.

smtpserver/server.go

@@ -1755,7 +1755,7 @@ func (c *conn) submit(ctx context.Context, recvHdrFor func(string) string, msgWr
 	if len(dkimConfig.Sign) > 0 {
 		if canonical, err := mox.CanonicalLocalpart(msgFrom.Localpart, confDom); err != nil {
 			c.log.Errorx("determining canonical localpart for dkim signing", err, mlog.Field("localpart", msgFrom.Localpart))
-		} else if dkimHeaders, err := dkim.Sign(ctx, canonical, msgFrom.Domain, dkimConfig, c.smtputf8, dataFile); err != nil {
+		} else if dkimHeaders, err := dkim.Sign(ctx, canonical, msgFrom.Domain, dkimConfig, c.smtputf8, store.FileMsgReader(msgPrefix, dataFile)); err != nil {
 			c.log.Errorx("dkim sign for domain", err, mlog.Field("domain", msgFrom.Domain))
 			metricServerErrors.WithLabelValues("dkimsign").Inc()
 		} else {