Shared deck download removes all non-ASCII characters from filename

> The deck at https://ankiweb.net/shared/info/1521800111 1 (chosen for its name only) is called:
> Русский - Турецкий Rusça-Türkçe kelimeler(слов) 407
> _-__Rusa-Trke_kelimeler_407.apkg

Probable solution:

Content-Disposition: attachment; filename="fallback.txt"; filename*=UTF-8''%E4%B8%AD%E6%96%87.txt

When the AnkiWeb code was originally written, `filename*` had not yet been introduced.

Originally reported on https://forums.ankiweb.net/t/shared-deck-download-removes-all-non-ascii-characters/47617

Start at the AnkiWeb shared-deck download handler, which is not identified by file or test in the issue, and reproduce the download with the linked deck. Inspect the Content-Disposition response and verify that the completed download preserves non-ASCII characters through the UTF-8 filename while retaining a fallback filename.

Written by the indexing model from the issue text.