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._context = context;
    if (this._instance) {
      this._updateClassComponent(element.props, context);
    } else {
      if (shouldConstruct(element.type)) {
        this._instance = new element.type(
          element.props,
          context,
          this._updater,
        );
        if (element.type.hasOwnProperty('contextTypes')) {
          currentlyValidatingElement = element;
          checkPropTypes(
            element.type.contextTypes,
            context,
            'context',
            getName(element.type, this._instance),
            getStackAddendum,
          );
          currentlyValidatingElement = null;
        }
        this._mountClassComponent(element.props, context);
      } else {
        this._rendered = element.type(element.props, context);
      }
    }
    this._rendering = false;
    return this.getRenderOutput();
  }
  unmount() {