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

componentDidMount() {
          mountCount++;
        }
        click = () => {
          numClicks++;
        };
        render() {
          return (
            <span ref="span" onClick={this.click}>Name: {this.props.name}</span>
          );
        }
      }
      var element = document.createElement('div');
      ReactDOM.render(<TestComponent />, element);
      var lastMarkup = element.innerHTML;
      
      
      ReactDOM.render(<TestComponent name="x" />, element);
      expect(mountCount).toEqual(1);