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

ReactMarkupChecksum = require('ReactMarkupChecksum');
    ReactTestUtils = require('ReactTestUtils');
    ReactReconcileTransaction = require('ReactReconcileTransaction');
    ExecutionEnvironment = require('ExecutionEnvironment');
    ExecutionEnvironment.canUseDOM = false;
    ReactServerRendering = require('ReactServerRendering');
    var DOMProperty = require('DOMProperty');
    ID_ATTRIBUTE_NAME = DOMProperty.ID_ATTRIBUTE_NAME;
    ROOT_ATTRIBUTE_NAME = DOMProperty.ROOT_ATTRIBUTE_NAME;
  });
  describe('renderToString', () => {
    it('should generate simple markup', () => {
      var response = ReactServerRendering.renderToString(
        <span>hello world</span>
      );
      expect(response).toMatch(new RegExp(
        '<span ' + ROOT_ATTRIBUTE_NAME + '="" ' +
          ID_ATTRIBUTE_NAME + '="[^"]+" ' +
          ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+">hello world</span>'
      ));
    });
    it('should generate simple markup for self-closing tags', () => {
      var response = ReactServerRendering.renderToString(
        <img />
      );
      expect(response).toMatch(new RegExp(
        '<img ' + ROOT_ATTRIBUTE_NAME + '="" ' +
          ID_ATTRIBUTE_NAME + '="[^"]+" ' +
          ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+"/>'
      ));