[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.

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile