title: When mapping ko.mapping.fromJS values are null
body: <p>I'm working with knockoutjs and I'm trying to populate ViewModel instance from JSON data. According to knockoutjs documentation I may use this statement:</p>
  
  <pre><code>ko.mapping.fromJS(data, viewModel);
  </code></pre>
  
  <p>Here is my code :</p>
  
  <pre><code>var pledgeVM=function(){            
        this.name=ko.observable();
        this.Assets=ko.observableArray([]);
        this.StartEdit=function(assetModel){
        };
   };
          pledge = {"name":"Moses","Assets":[{"CityId":13,"commetns":null},{"CityId":14,"commetns":null}]};
          var pledgeVMinstance=new pledgeVM();
          ko.mapping.fromJS(pledge,pledgeVMinstance);
  </code></pre>
  
  <p>For some reason data not populated (<code>pledgeVMinstance.name()</code> is  undefined)
  unless I change the statement to:</p>
  
  <pre><code>ko.mapping.fromJS(pledge,{},pledgeVMinstance);
  </code></pre>
  
  <p>Maybe somebody can explain me why things going that way.</p>