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
}
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.
Len returns the number of headers set i . e . the number of times f is called in VisitAll .