title: prawn pdf rendering partial results in blank page
body: <p>I try to use rails prawn template style. In my main prawn document "order_summary", I want to render a prawn template like this:</p>
  
  <pre><code>pdf = Prawn::Document.new(:page_size =&gt; "A4")
  render :partial =&gt; "invoice"
  </code></pre>
  
  <p>The name of the partial is: _invoice.pdf.prawn and resides in the very same folder as the main document.</p>
  
  <p>When running it, it results in a blank page. The page was rendered successfully according to the logs:</p>
  
  <blockquote>
    <p>Rendered documents/_invoice.pdf.prawn (756.7ms) Rendered
    documents/order_summary.pdf.prawn (1100.9ms) Completed 200 OK in
    2323ms (Views: 1141.1ms | ActiveRecord: 0.4ms)</p>
  </blockquote>
  
  <p>If I run the partial as "invoice.pdf.prawn" (without rendering it through another file), it works fine so I assume there is no code error or sth.</p>
  
  <p>What do I do wrong?</p>