conn_opt.go
func OnSend(f func(*Request, *Response)) ConnOpt {
	return func(c *Conn) { c.onSend = append(c.onSend, f) }
}
func OnSend(f func(*Request, *Response)) ConnOpt {
	return func(c *Conn) { c.onSend = append(c.onSend, f) }
}
// OnSend causes all requests sent on conn to invoke f(req, nil) and
// all responses to invoke f(nil, resp),