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
  })
})
```
shouldComponentUpdate() {
        scuCounter++;
      }
      render() {
        return <div />;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    expect(scuCounter).toEqual(0);
    const instance = shallowRenderer.getMountedInstance();
    instance.forceUpdate();
    expect(scuCounter).toEqual(0);
  });
  it('should shallow render a functional component', () => {
    function SomeComponent(props, context) {
      return (
        <div>
          <div>{props.foo}</div>
          <div>{context.bar}</div>
          <span className="child1" />