title: Sending ETag with $resource
body: <p>Using Angular JS's $resource service, is there a way to specify the If-None-Match and ETag headers for the 2nd, 3rd, etc polls of a resource?</p>
  
  <pre><code>    var SummarySearch = $resource(&lt;&lt;apiurl&gt;&gt;,
          { },
          {
              get: {
                  method: 'GET',
                  headers: {
                      'x-header': 'id'
                  }
              }
          });
  </code></pre>
  
  <p>I've gotten this to work for setting a constant header (it's just <code>x-header: id</code> in this case), but I need something that varies per request, based on the ETag I last saw.</p>
  
  <p>Update:</p>
  
  <p><code>transformRequest</code> looked promising, but I don't have access to the data that I initiated the request with, or the URL where that data ended up. As far as I can tell I only have access to the body I'd be POSTing (undefined in my case since I'm doing a GET) and the headersGetter function.</p>