get writable () {
    // can't write any more after response finished
    // response.writableEnded is available since Node > 12.9
    // https://nodejs.org/api/http.html#http_response_writableended
    // response.finished is undocumented feature of previous Node versions
    // https://stackoverflow.com/questions/16254385/undocumented-response-finished-in-node-js
    if (this.res.writableEnded || this.res.finished) return false

    const socket = this.res.socket
    // There are already pending outgoing res, but still writable
    // https://github.com/nodejs/node/blob/v4.4.7/lib/_http_server.js#L486
    if (!socket) return true
    return socket.writable
  }
Checks if the request is writable.
Tests for the existence of the socket
as node sometimes does not set it.

@return {Boolean}
@api private