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
  })
})
```
if (this._instance) {
      if (typeof this._instance.componentWillUnmount === 'function') {
        this._instance.componentWillUnmount();
      }
    }
    this._context = null;
    this._element = null;
    this._newState = null;
    this._rendered = null;
    this._instance = null;
  }
  _mountClassComponent(props, context) {
    this._instance.context = context;
    this._instance.props = props;
    this._instance.state = this._instance.state || emptyObject;
    this._instance.updater = this._updater;
    if (typeof this._instance.componentWillMount === 'function') {
      const beforeState = this._newState;
      this._instance.componentWillMount();
      
      if (beforeState !== this._newState) {
        this._instance.state = this._newState || emptyObject;
      }
    }
    this._rendered = this._instance.render();
    
    
  }
  _updateClassComponent(props, context) {