function normalize(keys, values) {
  var normalized = {};

  forEach(keys, function (name) {
    var value = values[name];
    normalized[name] = (value != null) ? String(value) : null;
  });
  return normalized;
}
Normalizes a set of values to string or `null`, filtering them by a list of keys.

@param {Array} keys The list of keys to normalize/return.
@param {Object} values An object hash of values to normalize.
@return {Object} Returns an object hash of normalized string values.