public function agregarItem(productos $producto, $cantidad)
{
$this->array = unserialize($_SESSION['carro']);
$this->cantidad = unserialize($_SESSION['cantidad']);
$this->array[$producto->get_id()] = $producto;
$this->cantidad[$producto->get_id()] = $cantidad;
$_SESSION['cantidad'] = serialize($this->cantidad);
$_SESSION['carro'] = serialize($this->array);
}