smtp buildMessage(): flush quotedprintable writer buff explicitly

[ttys3]

I found this problem when I was refactoring the email provider code ( the PR which got rejected by umputun).

a simple test run under my local machine ( 6 core 12 thread CPU with 4.7GHz freq) failed, the test code looks like:

	htmlContent := "<strong>simple HTML body goes here</strong>"
	t.Logf("try send via SMTP from %s to %s", fromEmail, toEmail)

	msg, err := buildMessage(toEmail, htmlContent, "text/html")
	if err != nil {
		t.Error(err)
	}
        // now only got the headers, body is missing
	t.Logf("mail msg: %s", msg)
	if !strings.Contains(msg, htmlContent) {
		t.Errorf("BuildMessage lost body")
	}

look into quotedprintable.Close() function, you' find it implicitly called flush():

// Close closes the Writer, flushing any unwritten data to the underlying
// io.Writer, but does not close the underlying io.Writer.
func (w *Writer) Close() error {
	if err := w.checkLastByte(); err != nil {
		return err
	}

	return w.flush()
}

the old buggy code is

remark42/backend/app/notify/email.go

Lines 374 to 378 in cab3b8a

	defer qp.Close()
	m := buff.String()
	message += "\n" + m
	return message, nil
	}

it called buff.String() after defer qp.Close() but the buff actrully got flushed after the m := buff.String() call

[paskal]

LGTM.