Make single checkboxes more user friendly

[peteryates]

Currently it appears that when creating a single check box the hidden field that should be rendered automatically by the Rails #check_box helper we're wrapping isn't present in the output.

I'm not sure why this happens, if there's a good reason it's not (and not just a misconfiguration bug) then it's probably a good idea to add the hidden field in this library when multiple: false.

Currently this is kind of mitigated by a note in the guide, but that can definitely be improved 😅

= f.govuk_check_box(:awesome, true, multiple: false, label: -> { 'Awesome?' })

<div class="govuk-checkboxes__item">
  <input id="person-awesome-true-field" class="govuk-checkboxes__input" type="checkbox" value="true" checked="checked" name="person[awesome]">
  <label for="person-awesome-true-field" class="govuk-label govuk-checkboxes__label">Awesome?</label>
</div>

• Work out why with a single checkbox the hidden field that should be generated by Rails isn't there
• Fix it if we've done something wrong. If not, take care of rendering it ourselves
• Update the documentation accordingly

[peteryates]

So, it seems that Rails only renders the necessary hidden field when the false value is 0, when the helper's called with false the output is omitted 🤦

[3] pry(#<GOVUKDesignSystemFormBuilder::Elements::CheckBoxes::FieldsetCheckBox>)> @builder.check_box(@attribute_name, {}, @value, false)
=> "<input type=\"checkbox\" value=\"11\" name=\"person[projects]\" id=\"person_projects\" />"
[4] pry(#<GOVUKDesignSystemFormBuilder::Elements::CheckBoxes::FieldsetCheckBox>)> @builder.check_box(@attribute_name, {}, @value, 0)
=> "<input name=\"person[projects]\" type=\"hidden\" value=\"0\" /><input type=\"checkbox\" value=\"11\" name=\"person[projects]\" id=\"person_projects\" />"