The slapos default configuration decision:
- text/html does not have Last-Modified
  Because we can not calculate proper last-modified date when the page consists
  of several parts. (e.g.: Gadgets)
- resouces have 'Cache-Control: public'
  Because several versions of Firefox request 'public' to cache resources
  over SSL.
So web_checker.cfg respects the slapos default configuration decision.