Completes the asynchronous operation started in BeginDownloadStream.
Before calling BeginDownloadStream, you need to create a callback method that implements the AsyncCallback delegate. This callback method executes in a separate thread and is called by the system after BeginDownloadStream returns. The callback method must accept the IAsyncResult returned by the BeginDownloadStream method as a parameter. This parameter may then be passed to the EndDownloadStream method to complete the operation.