message: add test to Button
diff: @@ -15,6 +15,10 @@ describe('Button', () => {
  expect(render(<Button />).type).toEqual('button')
  })
  + it('respects the "is" prop', () => {
  + expect(render(<Button is="a" />).type).toEqual('a')
  + })
  +
  it('implements common system props', () => {
  expect(Button).toImplementSystemProps(COMMON)
  })
  