function stringifyArray(arr, prefix) {
  var ret = [];
  if (!prefix) throw new TypeError('stringify expects an object');
  for (var i = 0; i < arr.length; i++) {
    ret.push(stringify(arr[i], prefix + '[' + i + ']'));
  }
  return ret.join('&');
}
Stringify the given `arr`.

@param {Array} arr
@param {String} prefix
@return {String}
@api private