title: DevExtreme dxRadioGroup default selectedIndex issue
body: <p>Iam using DevExtreme 14.2 version, am not able to set default value on dxRadioGroup. consider the following code i don't know where i did mistake.</p>
  
  <p>HTML:</p>
  
  <pre><code>      &lt;div data-bind="dxRadioGroup: { dataSource: items, layout: 'horizontal', value: defaultvalue, valueExpr: 'this' }"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>JS:</p>
  
  <pre><code> dxSample_13_2.home = function (params) {
  
    var viewModel = {
      items: [
          { text: "Tea" },
          { text: "Coffee" },
          { text: "juice" }
      ],
      defaultvalue: ko.observable(items[2]),
    }
  
    return viewModel; 
   };
  </code></pre>