title: Rails3 form_for hidden_field undefined method 'merge'
body: <p>My attempt to place a hidden_field within a form_for is crashing within cucumber on an ActionView helper error. Something also about FixNum which escapes me since I haven't dug through the source code. My prices_controller shows this:</p>
  
  <pre><code> @price = Price.new
    @commodity = Commodity.find(params[:id])
  </code></pre>
  
  <p>I want to make the link between price and commodity with this hidden_field:</p>
  
  <pre><code> &lt;%= form_for (@price), :url =&gt; prices_path  do |f| %&gt;
    &lt;% f.hidden_field :commodity_id, @commodity.id %&gt;
   .
   .
   &lt;div class="actions"&gt;
   &lt;%= f.submit "Submit" %&gt;
     &lt;/div&gt;
  </code></pre>
  
  <p>Looked at the form_for api and the above should work.  Reading other replies on stackoveflow, I have put the hidden_field in its own div within the form, added a Hidden_field_tag, and placed it within the actions div before the submit line. Looking at the merge msg, I guess it doesn't like something about the line, but it appears OK to me. The commodity_id field is the match field, sam</p>