Remove % from filenameFallback in Response::download

The filename fallback is turned to ASCII, because only ASCII is allowed. But there are 2 more checks:
 - check for `%` in the fallback
 - check for slashes in the filename and the fallback.

(See https://github.com/symfony/symfony/blob/v2.5.5/src/Symfony/Component/HttpFoundation/ResponseHeaderBag.php#L264-L277)

I think we can skip the slashes, because they are also checked in the filename itself. But we should probably remove the % sign. Downloads work when files have a `%` in them, just the fallback doesn't.

I propose removing the % from the fallback.