Closing `server` sets `websocket.readyState` to `undefined`

[mattnibs]

For version 2.0.0:

If you call server.close() somehow websocket.readyStategets set to undefined. readyState should be set to 3 or WebSocket.CLOSED.

I wrote a little mocha test to verify this bug:

it.only('calling close on server sets client readyState to WebSocket.CLOSED', done => {
    const server = new Server('ws://localhost:8080');
    const socket = new WebSocket('ws://localhost:8080');

    socket.onopen = function open() {
      server.close();
    }

    socket.onclose = function close() {
      assert.equal(socket.readyState, WebSocket.CLOSED);
      done();
    }
  });