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
  })
})
```
const oldProps = this._instance.props;
    if (
      oldProps !== props &&
      typeof this._instance.componentWillReceiveProps === 'function'
    ) {
      this._instance.componentWillReceiveProps(props, context);
    }
    
    
    const state = this._newState || this._instance.state || emptyObject;
    if (typeof this._instance.shouldComponentUpdate === 'function') {
      if (
        this._forcedUpdate ||
        this._instance.shouldComponentUpdate(props, state, context) === false
      ) {
        this._instance.context = context;
        this._instance.props = props;
        this._instance.state = state;
        this._forcedUpdate = false;
        return;
      }
    }
    if (typeof this._instance.componentWillUpdate === 'function') {
      this._instance.componentWillUpdate(props, state, context);
    }