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
  })
})
```
);
    expect(componentDidUpdateParams).toEqual([]);
    expect(componentWillReceivePropsParams).toEqual([]);
    expect(componentWillUpdateParams).toEqual([]);
    expect(setStateParams).toEqual([]);
    expect(shouldComponentUpdateParams).toEqual([]);
    
    shallowRenderer.render(
      React.createElement(SimpleComponent, updatedProp),
      updatedContext,
    );
    expect(componentWillReceivePropsParams).toEqual([
      updatedProp,
      updatedContext,
    ]);
    expect(setStateParams).toEqual([initialState, initialProp]);
    expect(shouldComponentUpdateParams).toEqual([
      updatedProp,
      updatedState,
      updatedContext,
    ]);
    expect(componentWillUpdateParams).toEqual([
      updatedProp,
      updatedState,
      updatedContext,
    ]);
    expect(componentDidUpdateParams).toEqual([]);
  });
  it('can shallowly render components with ref as function', () => {
    class SimpleComponent extends React.Component {