Remove % from filenameFallback in Response::download

Because it's not allowed in the fallback name.
(See https://github.com/symfony/symfony/blob/v2.5.5/src/Symfony/Component/HttpFoundation/ResponseHeaderBag.php#L264-L277)
Merge pull request #6097 from barryvdh/patch-4

[4.2] Remove % From filenameFallback In Response::download