title: codeigniter set_checkbox problem on an optional input
body: <p>i have an optional checkbox called checkall</p>
  
  <pre><code>&lt;input name="checkall" type="checkbox" value="ON" &lt;?php echo set_checkbox('checkall', 'ON'); ?&gt; /&gt;
  </code></pre>
  
  <p>i'm sure that the form is submitting that</p>
  
  <p>if i give it a validation rule <code>$this-&gt;form_validation-&gt;set_rules('checkall', 'Checkall', 'required');</code>it works, but without a rule nothing worked out !</p>
  
  <p>did i miss something? i think form helper doesn't require that for this function to work right ?</p>