Break reference cycle between WebSocketHandler and WebSocketProtocol on close.

This isn't strictly necessary, but it allows the refcounting CPython GC
to clean things up without waiting for a full GC.

Closes #382.

tornado/websocket.py

@@ -210,6 +210,7 @@ class WebSocketHandler(tornado.web.RequestHandler):
         Once the close handshake is successful the socket will be closed.
         """
         self.ws_connection.close()
+        self.ws_connection = None
 
     def allow_draft76(self):
         """Override to enable support for the older "draft76" protocol.