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
  })
})
```
this._instance.context = context;
    this._instance.props = props;
    this._instance.state = state;
    this._rendered = this._instance.render();
    
    
  }
}
class Updater {
  constructor(renderer) {
    this._renderer = renderer;
  }
  isMounted(publicInstance) {
    return !!this._renderer._element;
  }
  enqueueForceUpdate(publicInstance, callback, callerName) {
    this._renderer._forcedUpdate = true;
    this._renderer.render(this._renderer._element, this._renderer._context);
    if (typeof callback === 'function') {
      callback.call(publicInstance);
    }
  }
  enqueueReplaceState(publicInstance, completeState, callback, callerName) {
    this._renderer._newState = completeState;
    this._renderer.render(this._renderer._element, this._renderer._context);
    if (typeof callback === 'function') {
      callback.call(publicInstance);
    }
  }
  enqueueSetState(publicInstance, partialState, callback, callerName) {