title: Check file checksum before deciding to download
body: <p>I want to implement the following logic: fetch the big file (up to 100MB) and store it in the service worker. Then I want to check periodically (say once per hour) and if the remote file changes - need to re-download it. 
  Otherwise - I don't want to disturb the server.</p>
  
  <p>I was thinking about sending something like 'OPTIONS' request, just to get checksum of the file.</p>
  
  <p>How could I tackle this?</p>