docs: function(filterIndex, options) {
        if (options === undefined) {
          throw new Error('Request filter at index ' + filterIndex + ' returned nothing; it must return the filtered request options');
        } else if (!_.isObject(options)) {
          throw new Error('Expected request filter at index ' + filterIndex + ' to return the request options as an object, got ' + typeof(options));
        }
  
        return options;
      }