title: kendo ui grid filter doesn't work for integer type columns
body: <p>I have a problem with Kendo UI. I can't filter on some columns.</p>
  
  <p>Here's a <a href="http://plnkr.co/edit/OfzaIlyRp299sT3Qe3SS" rel="noreferrer">plunker</a> with my example where I try to filter on the columns id, prog or Max Temps and it doesn't work.</p>
  
  <pre><code>&lt;body&gt;
      &lt;div id="grid"&gt;&lt;/div&gt;
  &lt;/body&gt;
  &lt;script&gt;
      $("#grid").kendoGrid({
          dataSource: [
              { id: 36308, reportDate: "2015-02-01", prog: 58, state: "Waiting", maxTemps: 0 }, 
              { id: 36309, reportDate: "2015-02-01", prog: 34, state: "Complete", maxTemps: 86400 },
              { id: 36310, reportDate: "2015-02-01", prog: 116, state: "Complete", maxTemps: 86400  },
              { id: 36311, reportDate: "2015-02-02", prog: 58, state: "Complete", maxTemps: 86400 }
          ],
          filterable: true,
          columnMenu: true,
          columns: [
              { field: 'id', title: 'Id', width: '80px' },
              { field: 'reportDate', title: 'Report Date', width: '100px' },
              { field: 'prog', title: 'Prog', width: '60px' },
              { field: 'state', title: 'Status', width: '130px' },
              { field: 'maxTemps', title: 'Max Temps', width: '100px' }
          ]
      });
  &lt;/script&gt;
  </code></pre>
  
  <p>I have this error on Chrome: </p>
  
  <blockquote>
    <p>Uncaught TypeError: (d.prog || "").toLowerCase is not a function</p>
  </blockquote>
  
  <p>and this one on Firefox: </p>
  
  <blockquote>
    <p>TypeError: "".toLowerCase is not a function.</p>
  </blockquote>
  
  <p>I think it's because my data type is integer on these columns. I don't know how to solve this. Any ideas?</p>