add checkbox hidden field id if provided

Should not change the behaviour of the checkbox. 

Will create (2) instead of (1) when no `id` is provided:
(1)
```html
<input class="input--hidden" id="--hidden" name="sailthru[required][Opt_in]" type="hidden">
```
(2)
```html
<input class="input--hidden" name="sailthru[required][Opt_in]" type="hidden">
```