code: function formatRequestString_(o) {
    var ret;
    if (typeof o === 'object') {
      if (isArray_(o)) {
        ret = [];
        for (var i = 0, I = o.length; i < I; i++) {
          ret.push(formatRequestString_(o[i]));
        }
        return '[' + ret.join(',') + ']';
      } else if (isOverlay_(o)) {
        return fromOverlaysToJSON_(o);
      } else if (o.toJSON) {
        return o.toJSON();
      } else {
        ret = '';
        for (var x in o) {
          if (o.hasOwnProperty(x)) {
            if (ret.length > 0) {
              ret += ', ';
            }
            ret += x + ':' + formatRequestString_(o[x]);
          }
        }
        return '{' + ret + '}';
      }
    }
    return o.toString();
  }
docs: get string as rest parameter
  @param {Object} o