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.isRequired,
      };
      render() {
        return <div>{this.context.name}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    expectDev(console.error.calls.count()).toBe(1);
    expect(
      console.error.calls.argsFor(0)[0].replace(/\(at .+?:\d+\)/g, '(at **)'),
    ).toBe(
      'Warning: Failed context type: The context `name` is marked as ' +
        'required in `SimpleComponent`, but its value is `undefined`.\n' +
        '    in SimpleComponent (at **)',
    );
  });
  it('should warn about propTypes (but only once)', () => {
    spyOn(console, 'error');
    class SimpleComponent extends React.Component {