title: jqgrid sending empty ("")value with stype:'select' and searchoptions:{dataUrl:...} if user submits the search form without changing the selected value
body: <p>I'm using jqgrid with some column like this:</p>
  
  <pre><code>{label:'test',
  name:'dn',
  index:'dn_id',
  sortable:false,
  align:'center',
  editable:false,
  search:true,stype:'select',
  searchoptions:{
      sopt: ['eq'],
      defaultValue:'1000',
      dataUrl:'&lt;?php echo URL::base().'reports_dn/get_dns';?&gt;'}
  }
  </code></pre>
  
  <p>the select dropdown is created successfully from dataUrl. but if user selects search and presses submit button without changing the current dropdown selection, empty value is sent to server for 'dn_id'. but if I change the drop down selected value, it works correctly. it seems like a bug but I'm not sure about it. please help.
  thanx</p>