Allow checkbox without an assigned value attribute (in Rack_Test)

On different browsers,

```html
<input type="checkbox" name="foobar">
```

Will submit different values during form submission if no value="" attribute has been provided. Typically Firefox and Chrome supply the value "on" and this pull request makes the behaviour of Rack_Test consistent with those browsers.

http://stackoverflow.com/questions/10832640/input-checkbox-true-or-checked-or-yes
http://www.w3.org/TR/1999/REC-html401-19991224/interact/forms.html#h-17.4

To verify: setting the @javascript tag in cucumber resolved this, because Firefox was then used to render the page. However, I would prefer to run the cucumber test in a headless mode (for performance reasons).

Cheers,

Nigel