WL-2194 Encode UTF-8 in Content Disposition correctly

UTF-8 encoding and fallback ISO-8859-1 encoding should be provided.

    E.g. Content-Disposition: attachment;
                              filename="foo-ae.html";
                              filename*=UTF-8''foo-%c3%a4.html

See http://greenbytes.de/tech/tc2231/#encoding-2231-fb