[libp2p-ping] Properly close the substream in the InboundUpgrade. (#1606)

Not doing so may result in FIN/RST frames being sent before
earlier data frames. See https://github.com/paritytech/yamux/pull/81#issuecomment-643169973.

protocols/ping/src/protocol.rs

@@ -69,6 +69,7 @@ where
             while let Ok(_) = socket.read_exact(&mut payload).await {
                 socket.write_all(&payload).await?;
             }
+            socket.close().await?;
             Ok(())
         }.boxed()
     }