function stringifyObject(obj, prefix) {
  var ret = []
    , keys = objectKeys(obj)
    , key;

  for (var i = 0, len = keys.length; i < len; ++i) {
    key = keys[i];
    if ('' == key) continue;
    if (null == obj[key]) {
      ret.push(encodeURIComponent(key) + '=');
    } else {
      ret.push(stringify(obj[key], prefix
        ? prefix + '[' + encodeURIComponent(key) + ']'
        : encodeURIComponent(key)));
    }
  }

  return ret.join('&');
}
Stringify the given `obj`.

@param {Object} obj
@param {String} prefix
@return {String}
@api private