public async Task WhenCartExists_ReturnsCart(Cart cart)
{
this.repository.GetCart(Arg.Is <Guid>(p => p == cart.Id)).Returns(cart);
var result = await this.client.GetAsync($"/v1/carts/{cart.Id}");
AssertExt.ReturnsStatusCode_Ok(result);
await AssertExt.ReturnsCartReturnedFromRepository(cart, result);
}