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

                await AssertEx.Throws<ArgumentException>(async () => await client.StarRepo(null, "james"));
                await AssertEx.Throws<ArgumentException>(async () => await client.StarRepo("james", null));
            }
            public async Task ChecksStarredForUser()
            {
                var gitHubClient = Substitute.For<IGitHubClient>();
                var client = new ObservableStarredClient(gitHubClient);

                client.StarRepo("jugglingnutcase", "katiejamie");
                gitHubClient.Activity.Starring.Received().StarRepo("jugglingnutcase", "katiejamie");
            }
            public async Task EnsuresArguments()
            {
                var client = new ObservableStarredClient(Substitute.For<IGitHubClient>());

                await Assert.ThrowsAsync<ArgumentNullException>(() => client.StarRepo(null, "james").ToTask());
                await Assert.ThrowsAsync<ArgumentNullException>(() => client.StarRepo("james", null).ToTask());
            }