title: What does "WARN Could not determine content-length of response body." mean and how to I get rid of it?
body: <p>Since upgrading to Rails 3.1 I'm seeing this warning message in my development log:</p>
  
  <blockquote>
    <p>WARN  Could not determine content-length of response body. Set content-length of the response or set <code>Response#chunked = true</code></p>
  </blockquote>
  
  <p>What does this mean and how can I remove it? Is it a problem?</p>