title: Hidden inputs in formtastic
body: <p>I'm trying to add a hidden input with a predetermined "value" in it</p>
  
  <pre><code>&lt;%= f.input :step, :as =&gt; :hidden, :input_html =&gt; { :value =&gt; "3" } %&gt;
  </code></pre>
  
  <p>In this instance I'm using hidden inputs to determined which form on a page with more than one form is submitted.</p>
  
  <p>This looks to display correctly in the html but submitting the form produces this error</p>
  
  <blockquote>
    <p>unknown attribute: step</p>
  </blockquote>