subject doesn't follow preview updates

how to reproduce it:
- disable comment title
- create a comment with the text "abc def"
- preview the comment
- previewed subject is "abc def" (as it should)
- edit the comment body to read "123 def"
- re-preview the comment
- previewed subject still is "abc def" (should be "123 def")

cause:
- comment_form_build_preview doesn't takes into account that having #access=FALSE preserves the #default_value of the first preview in subsequent previews
- previously in D6 the subject field didn't existed when disabled, therefore comment_form_add_preview always had an empty subject and previewed subject was regenerated every time

suggestion:
- if #access==FALSE drop the $form_state['values']['subject'] when building the preview

function comment_form_build_preview($form, &$form_state) {
  if (!$form['subject']['#access']) {
    $form_state['values']['subject'] = ''; // <<-- it shouldn't be preserved between subsequent previews
  } 
  $comment = comment_form_submit_build_comment($form, $form_state);
  $form_state['comment_preview'] = comment_preview($comment);
}

relevance:
- this bug propagates to every module providing default values, therefore, previewing what will become the actual subject wouldn't be possible at all