header.go
func (h *RequestHeader) Cookie(key string) []byte {
	h.parseRawHeaders()
	h.collectCookies()
	return peekArgStr(h.cookies, key)
}
func (h *RequestHeader) Cookie(key string) []byte {
	h.parseRawHeaders()
	h.collectCookies()
	return peekArgStr(h.cookies, key)
}
// Cookie returns cookie for the given key.