**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 = initialSettings;
      shouldComponentUpdate(nextProps, nextState) {
        return shallowCompare(this, nextProps, nextState);
      }
      render() {
        renderCalls++;
        return <div />;
      }
    }
    var container = document.createElement('div');
    var instance = ReactDOM.render(<Component />, container);
    expect(renderCalls).toBe(1);
    
    var settings = {
      foo: initialSettings.foo,
      bar: initialSettings.bar,
    };
    instance.setState(settings);
    expect(renderCalls).toBe(1);
    
    initialSettings.foo = [1, 2, 3];
    instance.setState(initialSettings);
    expect(renderCalls).toBe(2);
    
    instance.setState(getInitialState());
    expect(renderCalls).toBe(3);
  });
});