[background] update delegate implementation

fixes https://github.com/MicrosoftDocs/xamarin-docs/issues/2420 from docs

SimpleBackgroundTransfer/SimpleBackgroundTransfer/SimpleBackgroundTransferViewController.cs

@@ -70,8 +70,8 @@ namespace SimpleBackgroundTransfer {
 		}
 	}
 
-	public class UrlSessionDelegate : NSUrlSessionDownloadDelegate
-	{
+	public class UrlSessionDelegate : NSObject, INSUrlSessionDownloadDelegate
+    {
 		public SimpleBackgroundTransferViewController controller;
 
 		public UrlSessionDelegate (SimpleBackgroundTransferViewController controller)
@@ -79,7 +79,7 @@ namespace SimpleBackgroundTransfer {
 			this.controller = controller;
 		}
 
-		public override void DidWriteData (NSUrlSession session, NSUrlSessionDownloadTask downloadTask, long bytesWritten, long totalBytesWritten, long totalBytesExpectedToWrite)
+		public void DidWriteData (NSUrlSession session, NSUrlSessionDownloadTask downloadTask, long bytesWritten, long totalBytesWritten, long totalBytesExpectedToWrite)
 		{
 			Console.WriteLine ("Set Progress");
 			if (downloadTask == controller.downloadTask) {
@@ -91,7 +91,7 @@ namespace SimpleBackgroundTransfer {
 			}
 		}
 
-		public override void DidFinishDownloading (NSUrlSession session, NSUrlSessionDownloadTask downloadTask, NSUrl location)
+		public void DidFinishDownloading (NSUrlSession session, NSUrlSessionDownloadTask downloadTask, NSUrl location)
 		{
 			Console.WriteLine ("Finished");
 			Console.WriteLine ("File downloaded in : {0}", location);
@@ -121,7 +121,7 @@ namespace SimpleBackgroundTransfer {
 			}
 		}
 
-		public override void DidCompleteWithError (NSUrlSession session, NSUrlSessionTask task, NSError error)
+		public void DidCompleteWithError (NSUrlSession session, NSUrlSessionTask task, NSError error)
 		{
 			Console.WriteLine ("DidComplete");
 			if (error == null)
@@ -137,12 +137,12 @@ namespace SimpleBackgroundTransfer {
 			controller.downloadTask = null;
 		}
 
-		public override void DidResume (NSUrlSession session, NSUrlSessionDownloadTask downloadTask, long resumeFileOffset, long expectedTotalBytes)
+		public void DidResume (NSUrlSession session, NSUrlSessionDownloadTask downloadTask, long resumeFileOffset, long expectedTotalBytes)
 		{
 			Console.WriteLine ("DidResume");
 		}
 
-		public override void DidFinishEventsForBackgroundSession (NSUrlSession session)
+		public void DidFinishEventsForBackgroundSession (NSUrlSession session)
 		{
 			using (AppDelegate appDelegate = UIApplication.SharedApplication.Delegate as AppDelegate) {
 				var handler = appDelegate.BackgroundSessionCompletionHandler;