Send a 0 value when a checkbox is unchecked

Someone [on the forum](http://octobercms.com/plugin/flynsarmy-menu/disabled-menuitems) noticed my checkboxes weren't staying unchecked after saving a form. This is because when a checkbox is checked a '1' POST value is sent, but when unchecked no value at all is sent and so the field isn't updated in the database. This PR sends a 0 unless the field is checked.