React has broken toggle-all checkbox

The React example has a broken toggle-all checkbox.  To reproduce the bug: 1) add two todo items, 2) set the first item to done, then 3) click toggle-all two times.  The first item will still be marked done, instead of both being marked not-done as should be.

The cause of the bug is from incorrectly setting checked to null instead of false.  From the React source code:

```
 * If `checked` or `value` are not supplied (or null/undefined), user actions
 * that affect the checked state or value will trigger updates to the element.
 *
 * If they are supplied (and not null/undefined), the rendered element will not
 * trigger updates to the element. Instead, the props must change in order for
 * the rendered element to be updated.
```

CC @chenglou