Field validation does not process hidden elements correctly

[pjdevries]

I ran into an issue with form field validation. This is what I see in .../media/system/js/fields/validate.js:

// Ignore the element if its currently disabled,
// because are not submitted for the http-request.
// For those case return always true.
if (element.getAttribute('disabled') === 'disabled' || element.getAttribute('display') === 'none') {
  this.handleResponse(true, element);
  return true;
}

An element with display: none; is not the same as being disabled and will be submitted. Therefore it should be handled like any other input element.