title: Service worker making extra call
body: <p>I'm trying to work with service workers for caching and running into an issue with authenticated calls. I'm currently using <a href="https://css-tricks.com/serviceworker-for-offline/" rel="nofollow noreferrer">https://css-tricks.com/serviceworker-for-offline/</a> (<a href="https://raw.githubusercontent.com/chriscoyier/Simple-Offline-Site/master/js/service-worker.js" rel="nofollow noreferrer">https://raw.githubusercontent.com/chriscoyier/Simple-Offline-Site/master/js/service-worker.js</a>) as a template though I've seen the same behavior using Google's sw-toolbox.</p>
  
  <p>When making a request that relies particular header parameters for authentication, it seems like the browser makes an initial request that fails, then a successful request (see image). Any thought as to why this is happening? Any other details that might be helpful?</p>
  
  <p>Thanks in advance,
  Dan</p>
  
  <p><a href="https://i.stack.imgur.com/G9Ncw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/G9Ncw.png" alt="enter image description here"></a></p>