message: tidy up className logic in UnderlineNav
diff: @@ -12,13 +12,13 @@ export default function UnderlineNav(props) {
  const classes = classnames(className, 'UnderlineNav', align && `UnderlineNav--${align}`, full && 'UnderlineNav--full')
  const mappedChildren = React.Children.map(children, child => {
  - const {className, selected} = child.props
  + const {className = '', selected} = child.props
  const newProps = {className}
  // add the ITEM_CLASS to all children without one
  - if (!className || className.indexOf(ITEM_CLASS) === -1) {
  + if (!className || !className.includes(ITEM_CLASS)) {
  newProps.className = classnames(ITEM_CLASS, className)
  }
  - if (selected === true) {
  + if (selected === true && !className.includes(SELECTED_CLASS)) {
  newProps.className = classnames(newProps.className, SELECTED_CLASS)
  }
  // if this is a react-router NavLink (duck typing!),
  