Always return options to default

hasHtmlHeader and hasHtmlFooter must return to their default state each time handleOptions() is called. Otherwise, if you generate 2 pdf files within 1 request (1 with a footer and 1 without it will throw an error).

Consider the following:

```php
$snappyPdf->generateFromHtml($pdf1Html, $pdf1File, array(
    'page-size'    => 'A5',
    'margin-right' => 0,
    'margin-left'  => 0,
    'footer-html'  => $footer
), true);

$snappyPdf->generateFromHtml($pdf2Html, $pdf2File, array(
    'page-size'    => 'A5',
    'margin-right' => 0,
    'margin-left'  => 0
), true);

```

When the second pdf is generated the following error is generated:

```Notice: Undefined index: footer-html in /knplabs/knp-snappy/src/Knp/Snappy/Pdf.php line 66```

This is because the first file generated is changing the protected $hasHtmlFooter value to true but there is no footer available.