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.
 *
 */
import React from 'react';
import describeComponentFrame from 'shared/describeComponentFrame';
import getComponentName from 'shared/getComponentName';
import emptyObject from 'fbjs/lib/emptyObject';
import invariant from 'fbjs/lib/invariant';
import checkPropTypes from 'prop-types/checkPropTypes';
class ReactShallowRenderer {
  static createRenderer = function() {
    return new ReactShallowRenderer();
  };
  constructor() {
    this._context = null;
    this._element = null;
    this._instance = null;
    this._newState = null;
    this._rendered = null;
    this._rendering = false;
    this._forcedUpdate = false;
    this._updater = new Updater(this);
  }
  getMountedInstance() {