code: function (id, options, callback) {
              var args = arguments,
                  retry = false,
                  extension,
                  url,
                  handler;
  
              if (typeof options === 'function') {
                  callback = options;
                  extension = '';
              } else {
                  options = extend({}, options);
                  retry = options.retry;
                  extension = options.extension || '';
                  // add a . if there is an extension
                  if (extension && !/^\./.test(extension)) {
                      extension = '.' + extension;
                  }
              }
  
              retry = (retry === true) && function () {
                  this.getContent.apply(this, args);
              }.bind(this);
  
              handler = createResponseHandler(callback, [200, 202], true, retry);
  
              url = client.documentsURL + '/' + id + '/content' + extension;
              return req(url, handler);
          }
docs: Fetches a document in the form specified by `extension`, which can be `pdf` or `zip`.
  If an extension is not specified, the document’s original format is returned.
  @param   {string}   id                   The document uuid
  @param   {Object}   [options]            Content options
  @param   {boolean}  [options.retry]      Whether to retry the request after 'retry-after' seconds if the retry-after header is sent
  @param   {string}   [options.extension]  The document format to request
  @param   {Function} [callback]           A callback to call with the response (or error)
  @returns {Request}