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
  })
})
```
static contextTypes = {
        name: PropTypes.string,
      };
      render() {
        return <div>{this.context.name}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SimpleComponent />, {
      name: 'foo',
    });
    expect(result).toEqual(<div>foo</div>);
  });
  it('should track context across updates', () => {
    class SimpleComponent extends React.Component {