override a def based on tornado 4.0 release

override a def based on tornado 4.0 release:

Document: http://www.tornadoweb.org/en/stable/releases/v4.0.0.html

WebSocket connections from other origin sites are now rejected by default. To accept cross-origin websocket connections, override the new method WebSocketHandler.check_origin.

We'd better to override the check_origin, and return Ture in the scenario of cross-origin

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