title: Add If-None-Match header
body: <p>For the url /foo, my backend returns:</p>
  
  <pre><code>Cache-Control: no-cache
  ETag: someetag
  </code></pre>
  
  <p>The cache-control directive is intended for the browser (which mustn't cache the response), I'd like varnish to cache the response and for subsequent requests to always hit the backend with a <code>If-None-Match: someetag</code> header, and if the backend returns 304, return a cached object.</p>