Some options cannot be selected alone in <select multiple />

If option A has a value, which is a sub string of the value of option B. Select B will make A selected.

http://jsfiddle.net/HADj3/

```js
/** @jsx React.DOM */

var MultipleSelect = React.createClass({
  render: function() {
    return <select multiple style={{height: 100, width: 100}}>
      <option value='1'>A</option>
      <option value='11'>B</option>
      <option value='111'>C</option>
      <option value='2'>D</option>
      <option value='12'>E</option>
    </select>;
  }
});

React.renderComponent(<MultipleSelect />, mountNode);
```

Select 'B' will make 'A' selected.
Select 'C' will make 'A' and 'B' selected.
Select 'E' will make 'A' and 'D' selected.

This issue also affects a controlled `<select multiple />`.

EDIT: It seems the fix is easy. I have converted this issue into a PR.