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
  })
})
```
return <div />;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SomeComponent />);
    shallowRenderer.unmount();
    expect(componentWillUnmount).toBeCalled();
  });
  it('can shallow render to null', () => {
    class SomeComponent extends React.Component {
      render() {
        return null;
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SomeComponent />);
    expect(result).toBe(null);
  });
  it('can shallow render with a ref', () => {
    class SomeComponent extends React.Component {
      render() {
        return <div ref="hello" />;
      }
    }
    const shallowRenderer = createRenderer();
    
    shallowRenderer.render(<SomeComponent />);
  });
  it('lets you update shallowly rendered components', () => {
    class SomeComponent extends React.Component {