title: Stop Service Worker Fetch Event caching specific files
body: <p>I'm listening for fetch events in a Service Worker and would like to know when is the response to a fetch cached? </p>
  
  <p>1) When the Fetch Event is triggered? PreventDefault() to stop it? 
  2) Do I need cache: no-store on my INIT parameter to my FETCH call? 
  3) Does it happen at event.respondWith() time?</p>
  
  <p>I have a data delivery strategy that seeks to deliver data as fresh as possible and only go to cache if the network is slow. BUT even then leave the network fetch running to update the cache whenever it finishes.</p>
  
  <p>Without something like INIT "no-cache" aren't we at risk of the FETCH just reading cache anyway? </p>
  
  <p>I (wrongly?) assumed caching was opt-in and I had to manually update the cache with a PUT().</p>