Added is any cancellation token argument

Octokit.Tests/Helpers/Arg.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Threading;
 using NSubstitute;
 using Octokit.Internal;
 
@@ -51,5 +52,10 @@ namespace Octokit.Tests
         {
             get { return Arg.Is<Dictionary<string, string>>(d => d.Count == 0); }
         }
+
+        public static CancellationToken CancellationToken
+        {
+            get { return Arg.Any<CancellationToken>(); }
+        }
     }
 }