[web] allow configuration of the Content-disposition value
The `set_content_type` function now takes and optional `disposition` parameter
to control the value of this HTTP header.
Use of `Content-disposition: inline` with a filename parameter are valid, so
the presence of filename does not allows to choose between `attachment` and
`inline`
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html