**Do you want to request a *feature* or report a *bug*?**
bug

**What is the current behavior?**
In normal rendering, you can memoize a class-based component: https://jsfiddle.net/586ea3cx/

With the shallow renderer, it seems like you can't: https://jsfiddle.net/odj217Lv/1/

This might be blocking https://github.com/airbnb/enzyme/pull/1914, in which tests are failing with `Cannot call a class as a function`. (it's tough to repro stuff with the shallow renderer)
static contextTypes = {
        foo: PropTypes.string,
      };
      state = {
        bar: 'bar',
      };
      render() {
        return <div>{`${this.context.foo}:${this.state.bar}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    let result = shallowRenderer.render(<SimpleComponent />, {
      foo: 'foo',
    });
    expect(result).toEqual(<div>foo:bar</div>);
    const instance = shallowRenderer.getMountedInstance();
    instance.setState({bar: 'baz'});
    result = shallowRenderer.getRenderOutput();
    expect(result).toEqual(<div>foo:baz</div>);
  });
  it('should filter context by contextTypes', () => {
    class SimpleComponent extends React.Component {