public async Task EnsuresNonNullArguments()
            {
                var client = new ObservableFollowersClient(Substitute.For<IGitHubClient>());

                await Assert.ThrowsAsync<ArgumentNullException>(() => client.IsFollowing(null, "alfhenrik-test").ToTask());
                await Assert.ThrowsAsync<ArgumentException>(() => client.IsFollowing("", "alfhenrik-test").ToTask());
                await Assert.ThrowsAsync<ArgumentNullException>(() => client.IsFollowing("alfhenrik", null).ToTask());
                await Assert.ThrowsAsync<ArgumentException>(() => client.IsFollowing("alfhenrik", "").ToTask());
            }
            public void IsFollowingFromClientUserFollowers()
            {
                var githubClient = Substitute.For<IGitHubClient>();
                var client = new ObservableFollowersClient(githubClient);

                client.IsFollowing("alfhenrik", "alfhenrik-test");

                githubClient.User.Followers.Received()
                    .IsFollowing("alfhenrik", "alfhenrik-test");
            }