Add updateWithAct to render

[ccfz]

Summary

To test behaviour that only occurs when props are updated it is
necessary to call update on the test instance. However, if the
behaviour under test is triggered by a hook, e.g. useEffect then
the test will fail because the hook is only triggered after the test
has run.

#111

Test plan

describe('when FloatLabel becomes active', () => {
  it('updates the Text color from dark to light', () => {
    const wrapper = render(<FloatLabel active={false} />)

    expect(expect(text.props.style).toMatchObject({ color: 'white' });)

    act(() => wrapper.update(<FloatLabel active />));
   // with the changes it will simply work by calling update

    expect(text.props.style).toMatchObject({ color: 'white' });
  });
});

[thymikee]

Thank you! If you rebased to latest master, you'd see this is already fixed by #137 and released as 1.6.2 just now :)

Looking forward to future contributions!