some improvements to websocket plugin and bugfix

fix bugs: 
1. for mix sessions, when received data from saved session, we should call a set_opts, and update the old saved socket to the new socket;
2. decoded websocket close frame return {{close, Reason}, Left}, not {close, Reason};

improvements:
1. add tcp_closed event for saved session, update the old saved socket to none;
2. add 'ack' attribute for 'message' request of websocket;
3. add example for websocket session.