text option for a button input should use its value

``` HTML
<button id="one" value="1">one</button>
<input id="two" type="button" value="two">
```

should mean all these are true:

``` javascript
find('button#one').shouldHave({text: 'one');
find('button#one').shouldHave({value: 1});
find('input#two').shouldHave({text: 'two'});
find('input#two').shouldHave({value: 'two'});
```
