protected void Page_Load(object sender, EventArgs e)
    {
        Response.Cache.SetCacheability(HttpCacheability.ServerAndNoCache);
        Response.Cache.SetAllowResponseInBrowserHistory(false);
        Response.Cache.SetNoStore();


        LUser dato = new LUser();

        GridView1.DataSource = dato.ListaVentas();
        GridView1.DataBind();
    }