bug in file download link?

Hi,

I've hitted a bug into Archetypes/utils.py contentDispositionHeader
function leading to bad download file name when this one contains non
ascii char, but I've been very surprised since no one fixed that while 
the fix is so obvious. So I asked myself what's the trap... Actually
without this fix this function is missing all its job imo (though even
with it download name are not always understood by every browser). 
I've attached the patch, if no one quickly tell me "hey that was 
intentional because...", I'll check it in next week.

Cheers,
-- 
Sylvain Thénault                               LOGILAB, Paris (France)
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