Make sure to always forward on keep-alives from the client

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -61,6 +61,8 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             }
             player.setPing(System.currentTimeMillis() - lastPingSent);
             resetPingData();
+            player.getConnectedServer().getMinecraftConnection().write(packet);
+            return;
         }
 
         if (packet instanceof ClientSettings) {