Value of Content-Disposition header:
Content-Disposition: attachment; filename*=UTF-8''foo-a%cc%88.html
Test Description:
'attachment', specifying a filename of foo-ä.html
, using RFC2231 encoded UTF-8, but
choosing the decomposed form (lowercase a plus COMBINING DIAERESIS) --
on a Windows target system, this should be translated to the preferred
Unicode normal form (composed).
UA should offer to download the resource as "foo-ä.html".