code: function cleanKey(key, dtype) {
    return (
      key === null ? ESCAPED_KEYS.null :
      isUndefined(key) ? ESCAPED_KEYS.undefined :
      dtype === 'date' ? key.valueOf() :
      key
    );
  }
docs: Returns a clean version of the given 'key', transforming if the dtype doesn't hash well directly and escaping if necessary to avoid collisions for missing values