Breaking changes

• Dropped support for Node.js 6 (1e6999b).
• Dropped support for url.Url instances in the WebSocket constructor
  (692d7b4).
• The behavior of WebSocket#{p{i,o}ng,send}() has changed when the
  readyState attribute is not OPEN (#1532)
  • If the readyState attribute is CONNECTING, an exception is thrown.
  • If the readyState attribute is CLOSING or CLOSED
    • The bufferedAmount attribute is increased by the length of the data
      argument in bytes.
    • If provided, the callback function is called with an error.
    • No exception is thrown even if the callback function is not provided.