title: dynamic rails host name for routes
body: <p>I'm doing this in application controller:</p>
  
  <pre><code>before_filter :set_app_host
  
  def set_app_host
    Rails.application.routes.default_url_options[:host] = "some url"
  end
  </code></pre>
  
  <p>This is to have a dynamic host name that can show up in <code>link_to</code> helper method.
  However this is not working, and the original host name is showing instead. Any ideas?</p>