Fix for non-ascii filenames

[yalanyali]

Fixed "The header content contains invalid characters" error thrown by Express on non-ascii filenames according to: https://tools.ietf.org/html/rfc6266#section-6

[thrackle]

Thanks for the patch, @yalanyali! Looks like the tests are failing. If you want to resolve those I can merge, and/or I'll make a note to circle back and try to get those written and this merged a little later.

[yalanyali]

Well, I've changed the content-disposition string to support UTF-8 characters. Tests are failing because they're expecting a certain match. I'd have to change the tests in order for them to not fail, which is something I wouldn't do. I've been using my own branch of this library on my project and I feel like it's a decision the maintainer should make, so it's up to you. Thanks :)