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 = {
        counter: 0,
      };
      render() {
        instance = this;
        return (
          <button ref="button" onClick={this.onClick}>
            {this.state.counter}
          </button>
        );
      }
    }
    const shallowRenderer = createRenderer();
    let result = shallowRenderer.render(<SimpleComponent defaultCount={1} />);
    expect(result.props.children).toEqual(0);
    instance.setState((state, props) => {
      return {counter: props.defaultCount + 1};
    });
    result = shallowRenderer.getRenderOutput();
    expect(result.props.children).toEqual(2);
  });
  it('can setState with a callback', () => {
    let instance;
    class SimpleComponent extends React.Component {