message: add test for "to" prop duck-typing in UnderlineNavLink
diff: @@ -18,6 +18,16 @@ describe('Caret', () => {
  expect(render(<UnderlineNavLink selected />)).toEqual(render(<a className={`${ITEM_CLASS} ${SELECTED_CLASS}`} />))
  })
  + it('adds activeClassName={SELECTED_CLASS} when it gets a "to" prop', () => {
  + const Mock = jest.fn(() => <div />)
  + render(<UnderlineNavLink tag={Mock} to='#' />)
  + expect(Mock.mock.calls[0][0]).toEqual({
  + to: '#',
  + className: 'UnderlineNav-item no-underline',
  + activeClassName: 'selected'
  + })
  + })
  +
  it('respects margin utility prop', () => {
  expect(rendersClass(<UnderlineNavLink m={4} />, 'm-4')).toEqual(true)
  })
  