Added optional CancellationToken

Author: khellang

src/Microsoft.AspNet.Http.Abstractions/HttpResponse.cs

@@ -111,15 +111,16 @@ public abstract class HttpResponse
         /// Sends the given file using the SendFile extension.
         /// </summary>
         /// <param name="file">The file to send.</param>
+        /// <param name="cancellationToken"></param>
         /// <returns></returns>
-        public virtual Task SendFileAsync(IFileInfo file)
+        public virtual Task SendFileAsync(IFileInfo file, CancellationToken cancellationToken = default(CancellationToken))
         {
             if (file == null)
             {
                 throw new ArgumentNullException(nameof(file));
             }
 
-            return SendFileAsync(file, 0, null, CancellationToken.None);
+            return SendFileAsync(file, 0, null, cancellationToken);
         }
 
         /// <summary>