[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.

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/html/index.html
Files: /usr/share/doc/cubicweb-documentation/html/*