fixed errors in unit tests

Octokit.Reactive/Clients/ObservableReleasesClient.cs

@@ -191,6 +191,8 @@ namespace Octokit.Reactive
         /// <returns>The created <see cref="Release"/>.</returns>
         public IObservable<Release> Create(int repositoryId, NewRelease data)
         {
+            Ensure.ArgumentNotNull(data, "data");
+
             return _client.Create(repositoryId, data).ToObservable();
         }
 

Octokit.Tests/Reactive/ObservableReleasesClientTests.cs

@@ -203,7 +203,7 @@ namespace Octokit.Tests.Reactive
             }
 
             [Fact]
-            public void EnsuresArgumentsNotNull()
+            public void EnsuresNonNullArguments()
             {
                 var releasesClient = new ObservableReleasesClient(Substitute.For<IGitHubClient>());
                 var data = new NewRelease("fake-tag");