header.go
func (h *ResponseHeader) Len() int {
	n := 0
	h.VisitAll(func(k, v []byte) { n++ })
	return n
}
func (h *ResponseHeader) Len() int {
	n := 0
	h.VisitAll(func(k, v []byte) { n++ })
	return n
}
// Len returns the number of headers set,
// i.e. the number of times f is called in VisitAll.