make <option>foo</option> behave like <option value=foo>foo</option>

This is a quick fix to make `<select>` dropdowns with two-way bindings easier to create in the simple case where each option's `value` is the same as its HTML content. It's actually more in line with the way `<option` elements are treated in JavaScript by default...

```html
<select>
  <option>foo</option>
</select>
```

```js
document.querySelector( 'select' ).value; // 'foo'
document.querySelector( 'option' ).value; // 'foo'
```

...so it stands to reason that two-way bindings should work that way in Ractive.