code: function(params) {
        var args = params ? corbel.utils.clone(params) : {};
        args.url = this._buildUri(this.uri + '/access');
        args.method = corbel.request.method.GET;
        args.noRedirect = true;
  
        var that = this;
  
        return this.request(args).
        then(function(response) {
          return that.request({
            noRetry: args.noRetry,
            method: corbel.request.method.POST,
            contentType: 'application/x-www-form-urlencoded; charset=UTF-8',
            data:response.data,
            url: response.headers.location
          });
        });
      }
docs: Generates a JWT that contains the scopes of the actual user's assets and redirects to iam to upgrade user's token
  @memberof corbel.Assets.AssetsBuilder.prototype
  @return {Promise} Promise that resolves to a redirection to iam/oauth/token/upgrade or rejects with a {@link CorbelError}