[fix] fix NodeWebSocket declaration (#613)
[fix] fix NodeWebSocket declaration (#613)

NodeWebSocket was be required ONLY if global variables self and WebSocket were not defined.

Closes https://github.com/socketio/engine.io-client/issues/609
socketio/engine.io-client,Automattic/engine.io-client,socketio/engine.io-client,socketio/engine.io-client