Podcast RSS requests answer wrong to If-Modified-Since requests

How to reproduce

• Create podcast with an episode in Chrome
• Open Podcast RSS URL in second Browser (Firefox), keep Chrome window opened
• In OO add new podcast episode or edit episode
• In Firefox window reload URL (make sure it uses a last-modified request, can be checked in network debugger view in FF)
  => OO answers with "not modified", changes do not appear
• Shift reload in Firefox
  => Changes now appear