Fix receiving files when duplicate packets == 0

Acks weren't sent at all in case duplicate packets were configured to be zero.

src/server.rs

@@ -221,7 +221,7 @@ impl Server {
                 worker_options.block_size,
                 worker_options.timeout,
                 worker_options.window_size,
-                self.duplicate_packets,
+                self.duplicate_packets + 1,
             );
             worker.receive()
         };