Users downloading non-ASCII attachments would see garbled characters. When used with object storage, AWS S3 would return an InvalidArgument error: Header value cannot be represented using ISO-8859-1.

Per RFC 5987 and RFC 6266, Content-Disposition should be encoded properly. This commit takes the Rails 6 implementation of ActiveSuppport::Http::ContentDisposition (https://github.com/rails/rails/pull/33829) and ports it here.

However, due to gitlab-workhorse#207 (closed), the UTF-8 encoded Content-Disposition doesn't actually get sent to the user. This MR at least fixes AWS S3 access.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/47673