Print extensions attribute on open event.
Print extensions attribute on open event.

R=yhirano@chromium.org

Review URL: https://codereview.appspot.com/16280047
google/pywebsocket,googlearchive/pywebsocket,google/pywebsocket,GoogleChromeLabs/pywebsocket3,googlearchive/pywebsocket,GoogleChromeLabs/pywebsocket3,googlearchive/pywebsocket,GoogleChromeLabs/pywebsocket3,google/pywebsocket