public ActionResult Update_Quantity_Cart(FormCollection form)
{
GioHang giohang = Session["GioHang"] as GioHang;
string ma_monan = form["MA_MONAN"];
int quantity = int.Parse(form["Quantity"]);
giohang.Update_quantity(ma_monan, quantity);
return(RedirectToAction("ShowToCart", "GioHang"));
}