title: How do I set Expires: header when using send_data
body: <p>I have a method in my controller which uses send_data like this:</p>
  
  <pre><code>def show
    expires_in 10.hours, :public =&gt; true
    send_data my_image_generator, :filename =&gt; "image.gif", :type =&gt; "image/gif"
  end
  </code></pre>
  
  <p>Using expires_in results in headers being sent like this:</p>
  
  <pre><code>HTTP/1.1 200 OK
  Connection: close
  Date: Fri, 25 Jun 2010 10:41:22 GMT
  ETag: "885d75258e9306c46a5dbfe3de44e581"
  Content-Transfer-Encoding: binary
  X-Runtime: 143
  Content-Type: image/gif
  Content-Disposition: inline; filename="image.gif"
  Content-Length: 1277
  Cache-Control: max-age=36000, public
  </code></pre>
  
  <p>What I would like to do is add an header like <code>Expires: (some exact date)</code> to keep the user agent from revalidating. But I don't see how to make send_data set that header?</p>
  
  <p>I guess I <em>could</em> set it explicitly in the <code>response.headers</code> hash, but surely there must be a wrapper for that (or something)?</p>