There is no extra guard or clause in `enqueueForceUpdate` when calling `render` which means the `render` always calls `shouldComponentUpdate` which can block even forced re-render during tests.

This for example [breaks shallow rendering in Enzyme when using MobX and observables](https://github.com/airbnb/enzyme/issues/1267).

Minimal test case:

```javascript
import React from 'react'
import ShallowRenderer from 'react-test-renderer/shallow';
const renderer = new ShallowRenderer();

class TestComponent extends React.Component {
  render() { return <span/> }
}

describe('forceUpdate', () => {
  it('does not call shouldComponentUpdate', () => {
    let called = 0
    TestComponent.prototype.shouldComponentUpdate = () => called += 1
    const component = renderer.render(<TestComponent/>);
    renderer._instance.forceUpdate()
    expect(called).toBe(0) // <- this throws because called === 1
  })
})
```
render() {
        return React.createElement('div', null, this.props.name);
      }
    }
    SimpleComponent.propTypes = {
      name: PropTypes.string.isRequired,
    };
    const shallowRenderer = createRenderer();
    shallowRenderer.render(React.createElement(SimpleComponent, {name: 123}));
    expect(console.error.calls.count()).toBe(1);
    expect(
      console.error.calls.argsFor(0)[0].replace(/\(at .+?:\d+\)/g, '(at **)'),
    ).toBe(
      'Warning: Failed prop type: Invalid prop `name` of type `number` ' +
        'supplied to `SimpleComponent`, expected `string`.\n' +
        '    in SimpleComponent',
    );
  });
  it('should enable rendering of cloned element', () => {
    class SimpleComponent extends React.Component {