don't uri-encode filename twice

Hi, Tom!
Pulled an update of this lib recently and it had stopped working properly for me. The problem is encodeURIComponent() getting called twice on the same filename. I see your comment: "prevent signature fail in case file name has spaces", but can't really guess why can this happen (you will encode `to_sign` string anyway). I haven't much time now to test filenames with spaces though.. So just consider my pull request, because you obviously know things better.