function serialize(obj) {
  if (!isObject(obj)) return obj;
  var pairs = [];
  for (var key in obj) {
    if (null != obj[key]) {
      pushEncodedKeyValuePair(pairs, key, obj[key]);
        }
      }
  return pairs.join('&');
}
Serialize the given `obj`.

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