Given the following test case

``` jsx
function TestCmp() {
  return <span>{NaN}</span>;
}

ReactDOM.render(<TestCmp />, document.getElementById('app'));
```

Produces the following warning:

```
Warning: Component's children should not be mutated.
    in span (created by TestCmp)
    in TestCmp (created by Unknown)
```

This is probably caused by [this mutation check](https://github.com/facebook/react/blob/328fc75bc910032ff37dc9d93429fb14b476bd0d/src/renderers/shared/hooks/ReactChildrenMutationWarningHook.js#L34), which was introduced in 15.3.0, but it doesn't work for NaN, because NaN != NaN

getInner = () => {
        
        
        return <div title="inner" ref={(c) => this.innerRef = c} />;
      };
      render() {
        return (
          <Wrapper
            title="wrapper"
            ref={(c) => this.wrapperRef = c}
            getContent={this.getInner}
            />
        );
      }
      componentDidMount() {
        
        expect(this.wrapperRef.getTitle()).toBe('wrapper');
        expect(ReactDOM.findDOMNode(this.innerRef).title).toBe('inner');
        mounted = true;
      }
    }
    var instance = <Component />;
    instance = ReactTestUtils.renderIntoDocument(instance);
    expect(mounted).toBe(true);
  });
  it('should call refs at the correct time', () => {
    var log = [];
    class Inner extends React.Component {