header.go
func (h *RequestHeader) SetCookie(key, value string) {
	h.parseRawHeaders()
	h.collectCookies()
	h.cookies = setArg(h.cookies, key, value, argsHasValue)
}
func (h *RequestHeader) SetCookie(key, value string) {
	h.parseRawHeaders()
	h.collectCookies()
	h.cookies = setArg(h.cookies, key, value, argsHasValue)
}
// SetCookie sets 'key: value' cookies.