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
  })
})
```
constructor(props) {
        super(props);
        this.state = {
          bar: 'bar',
        };
      }
      render() {
        return <div>{`${this.props.foo}:${this.state.bar}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    let result = shallowRenderer.render(<SimpleComponent foo="foo" />);
    expect(result).toEqual(<div>foo:bar</div>);
    const instance = shallowRenderer.getMountedInstance();
    const cloned = React.cloneElement(instance, {foo: 'baz'});
    result = shallowRenderer.render(cloned);
    expect(result).toEqual(<div>baz:bar</div>);
  });
});