public void ShouldReturnAveragePriceForCart()
        {
            Product[] products =
            {
              new Product("Milk", 12.10m),
              new Product("Bread", 10.12m)
            };

            ShoppingCart cart = new ShoppingCart(products);
            Assert.AreEqual(11.11m, cart.CalculateAveragePriceForCart());
        }