message: tidy up className logic in UnderlineNavLink
diff: @@ -5,12 +5,12 @@ import {ITEM_CLASS, SELECTED_CLASS} from './UnderlineNav'
  import {mapWhitespaceProps} from './props'
  export default function UnderlineNavLink(props) {
  - const {children, className: defaultClassName, selected, tag: Tag, ...rest} = mapWhitespaceProps(props)
  -
  - const className = classnames(defaultClassName, ITEM_CLASS, selected && SELECTED_CLASS)
  + const {children, selected, tag: Tag, ...rest} = props
  + const {className} = mapWhitespaceProps(rest)
  + const classes = classnames(ITEM_CLASS, selected && SELECTED_CLASS, className)
  return (
  - <Tag className={className} {...rest}>
  + <Tag className={classes} {...rest}>
  {children}
  </Tag>
  )
  