Fix the echo server example to prevent ConnectionClosed error (#189)

Related #188

The echo server would generate ConnectionClosed error when the peer
close the connection normally. This is because that `tungstenite` crate
automatically reply Close message to the peer. Then StreamExt::forward()
also forwards the Close message after the connection is closed.

examples/echo-server.rs

@@ -46,9 +46,11 @@ async fn accept_connection(stream: TcpStream) {
     info!("New WebSocket connection: {}", addr);
 
     let (write, read) = ws_stream.split();
-    read.forward(write)
+    // We should not forward messages other than text or binary.
+    read.try_filter(|msg| future::ready(msg.is_text() || msg.is_binary()))
+        .forward(write)
         .await
-        .expect("Failed to forward message")
+        .expect("Failed to forward messages")
 }
 
 fn main() -> Result<(), Error> {