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 foo={1} />);
    
    
    expect(logs).toEqual(['componentWillMount']);
    logs.splice(0);
    const instance = shallowRenderer.getMountedInstance();
    instance.setState({});
    expect(logs).toEqual(['shouldComponentUpdate', 'componentWillUpdate']);
    logs.splice(0);
    shallowRenderer.render(<SomeComponent foo={2} />);
    
    expect(logs).toEqual([
      'componentWillReceiveProps',
      'shouldComponentUpdate',
      'componentWillUpdate',
    ]);
  });
  it('should only render 1 level deep', () => {
    function Parent() {