[downloadable] fix filename in HTTP header (closes #2522325, #2522324) Before this changeset we use the `filename` header with utf8 encoded filename all the time. However RFC6266 says: The parameters "filename" and "filename*" differ only in that "filename*" uses the encoding defined in [RFC5987], allowing the use of characters not present in the ISO-8859-1 character set ([ISO-8859-1]). Therefore, we alter the code to: 1. Use `filename` and `ascii` encoding whenever possible, 2. use `filename*` with `utf8` encoding otherwise (with a filename fallback for old browser) We also switch the `content-disposition` value to attachement if filename is specified, this will result as a mandatory download according to RFC6266. This mandatory download is the expected behavior. We changes the filename encoding to RFC5987 which is simpler, supported by all and modern browser (including IE from version 6) and does not suffer from the continuation issue. (see ticket #2522324 for details)

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doc/book usr/share/doc/cubicweb-documentation

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doc/html usr/share/doc/cubicweb-documentation

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debian/cubicweb-doc usr/share/doc-base/cubicweb-doc