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
  })
})
```
state = {clicked: false};
      handleUserClick = () => {
        this.setState({clicked: true});
      };
      render() {
        return (
          <div
            ref={() => {}}
            onClick={this.handleUserClick}
            className={this.state.clicked ? 'clicked' : ''}
          />
        );
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    let result = shallowRenderer.getRenderOutput();
    expect(result.type).toEqual('div');
    expect(result.props.className).toEqual('');
    result.props.onClick();
    result = shallowRenderer.getRenderOutput();
    expect(result.type).toEqual('div');
    expect(result.props.className).toEqual('clicked');
  });
  it('can setState in componentWillMount when shallow rendering', () => {
    class SimpleComponent extends React.Component {