title: update form field with js but not showing in submitted data
body: <p>Experimenting with JsHelper in CakePHP 2.0, I got a Request method to update the value of a form field with the results of an AJAX call - so far, so good. However, on inspecting the form field it says:</p>
  
  <pre><code>&lt;input type="hidden" id="JobsPartUnitcost" name="data[JobsPart][unitcost]"&gt;5.55&lt;/input&gt;
  </code></pre>
  
  <p>but when I copied it and first pasted it above it said</p>
  
  <pre><code>&lt;input type="hidden" id="JobsPartUnitcost" name="data[JobsPart][unitcost]"&gt;&lt;/input&gt;
  </code></pre>
  
  <p>and when I submitted the form the value was empty. Why is the browser showing the value but the underlying DOM not registering it?</p>
  
  <p>Using Mac/Safari, CakePHP 2.0, JQuery</p>
  
  <p><em><strong>EDIT</em></strong>
  As requested here is form data dump</p>
  
  <pre><code>    Array ( [JobsPart] =&gt; Array ( [job_id] =&gt; 1 [company_id] =&gt; 4 [part_id] =&gt; 2 [qty] =&gt; 3 [unitcost] =&gt; ) )
  </code></pre>
  
  <p>and here is AJAX code</p>
  
  <pre><code>$this-&gt;Js-&gt;get('#JobsPartPartId')-&gt;event('change',
      $this-&gt;Js-&gt;request(
          array(
              'controller'=&gt;'JobsParts',
              'action'=&gt;'getPart'
          ),
          array(
              'update'=&gt;'#JobsPartUnitcost',
              'async' =&gt; true,
              'method' =&gt; 'post',
              'dataExpression'=&gt;true,
              'data'=&gt; $this-&gt;Js-&gt;serializeForm(array(
                  'isForm' =&gt; false,
                  'inline' =&gt; true
              ))
          )
      )
  );
  </code></pre>