[web/httpcache] fix Last-Modified generation time.mktime takes a time tuple in *local* time. We have an UTC datetime coming from the last_modified method, which we were interpreting as local time, leading to hilarity. Use calendar.timegm instead. Add tests for Last-Modified generation of the 'download' and 'manage' views.

version=3
opts=uversionmangle=s/(rc|a|b|c)/~$1/ \
https://pypi.debian.net/cubicweb/cubicweb-(.+)\.(?:zip|tgz|tbz|txz|(?:tar\.(?:gz|bz2|xz)))