title: Rails 3 + JSON + Content Length not sent
body: <p>I'm using Rails for a WebApp (development with mongrel and production with nginx).
  I send JSOn data with:</p>
  
  <pre><code>respond_to do |format|
    format.json { render :json =&gt; { :success =&gt; "OK", :items =&gt; items } }
  end
  </code></pre>
  
  <p>It works. But I need to have the Content-Length, and it's not sent. How to force the Content-Length to be sent in headers when getting JSON ? Is-it a Rails config problem, or a server config problem ? </p>
  
  <p>Is there a way to set it in Rails ? Like :</p>
  
  <p>response['Content-Length'] = xxx</p>
  
  <p>Is it a good idea ? In my case I send a big array of JSON data (300 Kb).</p>