<input> tag generated by <s:checkbox> produce HTML warning in validators

<s:checkbox> tag with default xhtml theme produce an <input> tag, like that one:

<input type="hidden" id="_checkbox_is_vacancy_open" name="_checkbox_vacancy.open" value="true" />

produces HTML Tidy validator warning about an attribute name and ID starting with two "_" signs. So, here is a patch I suggest to resolve this issue:

I changed FreeMarker template and a checkbox interceptor code in such a way that prefix of that input no longer produce warning.