Handle new way of reporting unmodified FileFields when a FileField has not been modified, value is an empty string. Handle this is raise UnmodifiedField accordingly.

merge

attempt to port windows service to 3.8.1

[web session] fix potential key error on logout (occurs once the session has been transparently reconnected, hence session.sessionid and session.cnx.sessionid differs)

[css] update print css

[doc] cleanup

[python2.6] do not import simplejson directly use indirect import through cubicweb.web