var got strings.Builder

	req, _ := NewRequest("GET", cst.ts.URL, nil)
	req = req.WithContext(httptrace.WithClientTrace(context.Background(), &httptrace.ClientTrace{
		Got1xxResponse: func(code int, header textproto.MIMEHeader) error {
			fmt.Fprintf(&got, "1xx: code=%v, header=%v\n", code, header)
			return nil
		},
	}))
	res, err := cst.c.Do(req)
	if err != nil {
		t.Fatal(err)