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
  })
})
```
/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @emails react-core
 */
'use strict';
let createRenderer;
let PropTypes;
let React;
describe('ReactShallowRenderer', () => {
  beforeEach(() => {
    createRenderer = require('react-test-renderer/shallow').createRenderer;
    PropTypes = require('prop-types');
    React = require('react');
  });
  it('should call all of the lifecycle hooks', () => {
    const logs = [];
    const logger = message => () => logs.push(message) || true;
    class SomeComponent extends React.Component {
      componentWillMount = logger('componentWillMount');
      componentDidMount = logger('componentDidMount');
      componentWillReceiveProps = logger('componentWillReceiveProps');
      shouldComponentUpdate = logger('shouldComponentUpdate');
      componentWillUpdate = logger('componentWillUpdate');
      componentDidUpdate = logger('componentDidUpdate');
      componentWillUnmount = logger('componentWillUnmount');
      render() {