fix warning when using group on form-horizontal

`get_class` should only check the `$Args[1]`.

Found the bug in the example, using form-horizontal for the file field will generate warning because `$Args[$i]` when `$i === $ArgCount - 1` is a string (`"Help Text"`):

    $Form->group($Form->label('File'), new \JasonKaz\FormBuild\File(), "Help Text");