public async Task PostsToTheCorrectUrl()
            {
                var connection = Substitute.For <IApiConnection>();
                var client     = new ProductsClient(connection);

                await client.AddFollower(123, 1234);

                Received.InOrder(async() =>
                {
                    await connection.Post <ProductFollower>(Arg.Is <Uri>(u => u.ToString() == "products/123/followers"),
                                                            Arg.Is <object>(o => o.ToString() == new { user_id = 1234 }.ToString()));
                });
            }