Prepare requests for re-sending after a DC migration

Fixes #608.

TLSharp.Core/TelegramClient.cs

@@ -123,6 +123,8 @@ namespace TLSharp.Core
                 catch(DataCenterMigrationException e)
                 {
                     await ReconnectToDcAsync(e.DC);
+                    // prepare the request for another try
+                    request.ConfirmReceived = false;
                 }
             }
         }