title: rails 4.0 beta, fields_for not accepting more than one argument
body: <p>This works: </p>
  
  <pre><code>&lt;%= f.fields_for :exercise do |builder| %&gt;
              &lt;%= render 'exercise_fields', f: builder %&gt;
          &lt;% end %&gt;
          &lt;%= add_fields "Add Exercise", f, :exercises %&gt;
  
      &lt;%= f.submit %&gt;
  </code></pre>
  
  <p>but adding another argument (in this case a reference to the object) throws an ArgumentError:</p>
  
  <pre><code>&lt;% @workout.exercises.each do |exercise| %&gt;
  
          &lt;%= f.fields_for :exercise, exercise do |builder| %&gt;
              &lt;%= render 'exercise_fields', f: builder %&gt;
          &lt;% end %&gt;
          &lt;% end %&gt;
          &lt;%= add_fields "Add Exercise", f, :exercises %&gt;
  
      &lt;%= f.submit %&gt;
  </code></pre>
  
  <p>Error: wrong number of arguments (2 for 1)</p>
  
  <p>I've seen this multiple times in different places. Is this a bug or am I making some simple mistake?</p>