Handle connection closed by client

Author: fubar-coder

src/FubarDev.FtpServer.Abstractions/Commands/DefaultFtpCommandDispatcher.cs

@@ -7,6 +7,7 @@
 using System.ComponentModel.DataAnnotations;
 using System.Diagnostics;
 using System.Linq;
+using System.Net.Sockets;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -190,6 +191,9 @@ private async Task ExecuteCommandAsync(
                         _logger?.LogWarning(validationException.Message);
                         break;
 
+#if !NETSTANDARD1_3
+                    case SocketException se when se.ErrorCode == (int)SocketError.ConnectionAborted:
+#endif
                     case OperationCanceledException _:
                         response = new FtpResponse(426, localizationFeature.Catalog.GetString("Connection closed; transfer aborted."));
                         Debug.WriteLine($"Command {command} cancelled with response {response}");