Issue 432: Batch request with null callback throws exception

Src/GoogleApis/Apis/Requests/BatchRequest.cs

Index: Src/GoogleApis/Apis/Requests/BatchRequest.cs
===================================================================
--- a/Src/GoogleApis/Apis/Requests/BatchRequest.cs
 b/Src/GoogleApis/Apis/Requests/BatchRequest.cs
@@ -101,6 101,9 @@
 HttpResponseMessage message)
 {
 base.OnResponse(content, error, index, message);
 if (OnResponseCallback == null)
 return;
 OnResponseCallback(content as TResponse, error, index, message);
 }
 }
@@ -158,7 161,7 @@
 }
 /// <summary>Asynchronously executes the batch request.</summary>
- /// <param name="taskCancellationToken">Cancellation token to cancel operation.</param>
 /// <param name="cancellationToken">Cancellation token to cancel operation.</param>
 public async Task ExecuteAsync(CancellationToken cancellationToken)
 {
 if (Count < 1)