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

**What is the current behavior?**
In React v16, the callback function that is an optional parameter to the `setState` function is called and passed a value of null.  Previously, in React v15, the function was called with a value of undefined.  This change breaks the default parameters feature of JavaScript.

I discovered this behavior while working on a React Native project (hence the example using View & Text).

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template: https://jsfiddle.net/84v837e9/).**

```
import React, {
	Component,
} from 'react';

export default class MyComp extends Component {
	componentDidMount() {
		this.setState({}, (val) => console.log(val))
	}

	render() {
		return (
			<View>
				<Text>{this.props.climbs}</Text>
			</View>
		);
	}
}
```

**What is the expected behavior?**
The callback function will be called with undefined

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
Broken behavior was observed in React 16 alpha 12 and React Native 0.45.1.  

I replicated the issue in React 15.4.1 and React Native 0.40.0 and it was working as expected.
state = {value: 0};
      componentWillUnmount() {
        expect(() => {
          this.setState({value: 2}, function() {
            cbCalled = true;
          });
        }).not.toThrow();
      }
      render() {
        return <div />;
      }
    }
    var instance = ReactDOM.render(<Component />, container);
    instance.setState({value: 1});
    ReactDOM.unmountComponentAtNode(container);
    expect(cbCalled).toBe(false);
  });
  it('should warn about `setState` in render', () => {
    spyOn(console, 'error');
    var container = document.createElement('div');
    var renderedState = -1;
    var renderPasses = 0;
    class Component extends React.Component {