Returning plain string from component renders collapsed text nodes on server

Observed this on react@16.0.0-beta.5

```js
import React from 'react';
import { renderToString } from 'react-dom/server';

const B = () => 'b';

console.log(renderToString(<div>{'a'}<B />{'c'}</div>));
```

This renders as `<div data-reactroot="">abc</div>`. If you try to hydrate this you'll get the error: Text content did not match. Server: "bc" Client: "b".