cubicweb #2522324 downloadable file name are truncated [done]

We use the filename argument of the content-disposition header. we encode it in all case using the email module.

The email module add argument continuation in the encoding that are not supported for http

As a result, file name are truncated

done in3.15.6
load left0.000
closed by#d74addac92bb [downloadable] fix filename in HTTP header (closes #2522325, #2522324)