title: Kendo UI grid with hidden column and aggregate
body: <p>If I create a hidden column, in this case BirthDateMonth and create it from the dataset, if I also add a group aggregate on another field then it will break with JS error "<strong>sum not defined</strong>".</p>
  
  <pre><code>var grid = $("#grid").kendoGrid({
      dataSource: {
          data: createRandomData(10),
          schema: {
              model: {
                  fields: {
                      FirstName: { type: "string" },
                      LastName: { type: "string" },
                      City: { type: "string" },
                      Title: { type: "string" },
                      BirthDate: { type: "date" },
                      //BirthDateMonth: { type: "date" },
                      Age: { type: "number" }
                  },
              },
              parse: function (data) {
                  $.each(data, function (idx, item) {
                      if (item.BirthDate)
                      {
                          item.BirthDateMonth = new Date(item.BirthDate.getTime());
                          item.BirthDateMonth.setDate(1);  
                      }                        
                  });
                  return data;
              }                
          },
          pageSize: 10,
          aggregate: [        
                  {field: "Age", aggregate: "sum"}
          ]
      },
      height: 500,
      scrollable: true,
      groupable: true,
      columns: [
          {
              field: "FirstName",
              title: "First Name"
          },
          {
              field: "LastName",
              title: "Last Name"
          },
          {
              field: "City",
          },
          {
              field: "Title"
          },
          {
              field: "BirthDate",
              title: "Birth Date",
              template: '#= kendo.toString(BirthDate,"MM/dd/yyyy") #'
          },
          {
              field: "BirthDateMonth",
              title: "Birth Month",
              template: '#= kendo.toString(BirthDateMonth,"MM/yyyy") #',
              hidden: true
          },
          {
              field: "Age",
              aggregates: ["sum"],
              footerTemplate: "Sum: #=sum#",
              groupFooterTemplate: "Sum: #=sum#"               
  
          }
      ]
  }).data("kendoGrid");
  
  grid.dataSource.group([{field: "BirthDateMonth"}]);
  </code></pre>
  
  <p><a href="http://jsfiddle.net/uop9nyax/" rel="nofollow">JSFiddle</a>, any thoughts appreciated. I tried adding the hidden column field to the schema, but no effect.</p>