header.go
func (h *RequestHeader) Add(key, value string) {
	k := getHeaderKeyBytes(&h.bufKV, key, h.disableNormalizing)
	h.h = appendArg(h.h, b2s(k), value, argsHasValue)
}
func (h *RequestHeader) Add(key, value string) {
	k := getHeaderKeyBytes(&h.bufKV, key, h.disableNormalizing)
	h.h = appendArg(h.h, b2s(k), value, argsHasValue)
}
// Add adds the given 'key: value' header.
//
// Multiple headers with the same key may be added with this function.
// Use Set for setting a single header for the given key.