code: function getResponseStatusCode (res) {
    var status = res.statusCode
  
    // default status code to 500 if outside valid range
    if (typeof status !== 'number' || status < 400 || status > 599) {
      status = 500
    }
  
    return status
  }
docs: Get status code from response.
  
  @param {OutgoingMessage} res
  @return {number}
  @private