New convention test was added. Now each "TheCtor" class must have "EnsuresNonNullArguments" method were throwing of ArgumentNullException from client constructor class should be tested.

All unused "using" directives were removed.

Octokit.Tests/Reactive/ObservableUserKeysClientTests.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Linq;
 using NSubstitute;
 using Octokit.Reactive;
 using Xunit;
@@ -84,7 +83,7 @@ namespace Octokit.Tests.Reactive
         public class TheCtor
         {
             [Fact]
-            public void EnsuresArgument()
+            public void EnsuresNonNullArguments()
             {
                 Assert.Throws<ArgumentNullException>(() => new ObservableUserKeysClient(null));
             }