title: Downloading - get a callback when received all the request headers
body: <p>I use <a href="https://github.com/koush/ion" rel="nofollow">ion library</a> for downloading files in my app. I want to read the response headers, check a particular header and either cancel or continue the download. Of course I can execute the HEAD query, check the header, and then execute the GET query. But I want to execute only one request.</p>
  
  <p><strong>Is there a way to get a callback when received all the headers, handle them and either continue or cancel the download?</strong></p>