Adjust BaseWebDriver.select xpath to accomodate optgroup elements

Quick adjustment in case you have select fields like this:

```html
<select name="food">
  <optgroup label="Fruit">
    <option value="apple">Apple</option>
    <option value="grape">Grape</option>
  </optgroup>
  <optgroup label="Meat">
    <option value="beef">Beef</option>
  </optgroup>
</select>
```