title: Why browser does not send "If-None-Match" header?
body: <p>I'm trying to download (and hopefully cache) a dynamically loaded image in PHP. Here are the headers sent and received:</p>
  
  <p>Request:</p>
  
  <pre><code>GET /url:resource/Pomegranate/resources/images/logo.png HTTP/1.1
  Host: pome.local
  Connection: keep-alive
  Cache-Control: max-age=0
  Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
  User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.22 (KHTML, like Gecko) Ubuntu Chromium/25.0.1364.160 Chrome/25.0.1364.160 Safari/537.22
  Accept-Encoding: gzip,deflate,sdch
  Accept-Language: en-US,en;q=0.8
  Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
  Cookie: PHPSESSID=fb8ghv9ti6v5s3ekkmvtacr9u5
  </code></pre>
  
  <p>Response:</p>
  
  <pre><code>HTTP/1.1 200 OK
  Date: Tue, 09 Apr 2013 11:00:36 GMT
  Server: Apache/2.2.22 (Ubuntu)
  X-Powered-By: PHP/5.3.14 ZendServer/5.0
  Expires: Thu, 19 Nov 1981 08:52:00 GMT
  Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
  Pragma: no-cache
  Content-Disposition: inline; filename="logo"
  ETag: "1355829295"
  Last-Modified: Tue, 18 Dec 2012 14:44:55 Asia/Tehran
  Keep-Alive: timeout=5, max=98
  Connection: Keep-Alive
  Transfer-Encoding: chunked
  Content-Type: image/png
  </code></pre>
  
  <p>When I reload the URL, the exact same headers are sent and received. My question is what should I send in my response to see the <code>If-None-Match</code> header in the consequent request?</p>
  
  <p>NOTE: I believe these headers were doing just fine not long ago, even though I can not be sure but I think browsers are changed not to sent the <code>If-None-Match</code> header anymore (I used to see that header). I'm testing with Chrome and Firefox and both fail to send the header.</p>