code: function csrf(cb) {
    var self = this;
  
    this.opts.headers = this.opts.headers || {};
  
    function resultCb(err, data, response) {
      if (!err) {
        self.opts.headers[data.crumbRequestField] = data.crumb;
        self.opts.headers.jenkinsCrumb = data.crumb;
      }
      cb(err, data);
    }
  
    this.crumb(resultCb);
  }
docs: Add Jenkins crumb header to instance.
  This is needed for Jenkins installations that have CSRF protection enabled.
  New installations of Jenkins starting version 2.x enables CSRF protection by default.
  https://wiki.jenkins-ci.org/display/JENKINS/CSRF+Protection
  
  @param {Function} cb: standard cb(err, result) callback