title: Specifying format: "xml" ignored with render_to_string
body: <p>I have an action that needs to render a view to string. The view is called index.xml.erb. I am trying to achieve this with render_to_string:</p>
  
  <pre><code>my_string = render_to_string(layout: false, format: "xml")
  </code></pre>
  
  <p>render_to_string is instead rendering the contents of index.html.erb and assigning it to my_string. What am I missing?</p>
  
  <p><strong>Note</strong>: I am aware that I can do something like this:</p>
  
  <pre><code>my_string = render_to_string(:action =&gt; "#{self.action_name}.xml.erb")
  </code></pre>
  
  <p>But I'm curious as to why the "format" option isn't honored with render_to_string.</p>