Handle NoContent response for queued operations

We didn't handle the 204 response properly for the `StatisticsClient`.
Now we do.

Fixes #836

Octokit/Http/IApiConnection.cs

@@ -262,6 +262,6 @@ namespace Octokit
         /// <param name="cancellationToken">A token used to cancel this potentially long running request</param>
         /// <returns>The updated API resource.</returns>
         /// <exception cref="ApiException">Thrown when an API error occurs.</exception>
-        Task<T> GetQueuedOperation<T>(Uri uri,CancellationToken cancellationToken);
+        Task<T> GetQueuedOperation<T>(Uri uri, CancellationToken cancellationToken);
     }
-}
+}