title: setGroupHeaders of JQGrid for column headers grouping is not working
body: <p>I am using JQGrid to represent tabular data. I want to group 2 column headers. I am using setGroupHeaders method of JQGrid, but it's not working. Even there is no any error in console.
  Here is my code. Where am I going wrong? Thanks in advance.</p>
  
  <p>...</p>
  
  <pre><code>jQuery("#DataGridTable").jqGrid({
  
      url: '/Home/ViewDeferredAccount',
      contentType: "application/json",
      datatype: "json",
      colNames: ['Key','Sector Code', 'Route Code', 'EL1', 'EL2', 'Payment Cmp Code', 'Revenue Cmp Code'],
      colModel: [
  
      { name: 'DEFRD_REV_AC_KEY', index: 'DEFRD_REV_AC_KEY', width: 40, stype: 'text' },
      { name: 'SECTOR_CD', index: 'SECTOR_CD', width: 180, stype: 'text' },
      { name: 'ROUTE_CD', index: 'ROUTE_CD', width: 150 },
      { name: 'EL1_AC_CD', index: 'EL1_AC_CD', width: 140, sortable: false },
      { name: 'EL2_AC_CD', index: 'EL2_AC_CD', width: 140, sortable: false },
      { name: 'PYMN_CMP_CD', index: 'PYMN_CMP_CD', width: 180 },
      { name: 'RVN_CMP_CD', index: 'RVN_CMP_CD', width: 180 }
      ],
      rowNum: 10,
      viewrecords: true,
      loadonce: true,
      sortorder: "desc",
      caption: "Maintain and Setup Deferred Account",
  });
  </code></pre>
  
  <p>...</p>
  
  <pre><code>jQuery("#DataGridTable").jqGrid('setGroupHeaders', {
          useColSpanStyle: true,
          groupHeaders: [
            { startColumnName: 'EL1_AC_CD', numberOfColumns: 2, titleText: 'Sector Code' }
          ]
      });
  </code></pre>