title: paper-datatable not detecting the data-source/data-object defined locally in polymer-element
body: <p>I am trying to use paper-datatable-card in one of my polymer-elements. In data-source i have given a variable which is an array of object. </p>
  
  <pre><code>&lt;paper-datatable-card id="datatableCard" header="Checklist" page-size="10" data-source="{{users}}" id-property="id" selected-ids="{{selectedIds}}"&gt;
     &lt;paper-datatable id="datatable" selectable multi-selection selected-items="{{selectedItems}}"&gt;
          &lt;div no-results&gt;
            Loading or no more items...
          &lt;/div&gt;
          &lt;paper-datatable-column header="ID" property="id" type="String" default="1" tooltip="Some title" sortable align="center" sorted&gt;&lt;/paper-datatable-column&gt;
          &lt;paper-datatable-column header="Task Name" property="name" type="String" defualt="abc" tooltip="PM task" sortable&gt;&lt;/paper-datatable-column&gt;
     &lt;/paper-datatable&gt;      
  
  &lt;/paper-datatable-card&gt;
  </code></pre>
  
  <p>
  Below is the script of my polymer-element :</p>
  
  <pre><code>&lt; script &gt;
  
  
    Polymer({
      is: 'opt-main',
  
      properties: {
  
      },
  
      ready: function() {
        this.users = [
              {id: 0, name: 'Lily'},
              {id: 1, name: 'Emila'}
          ];
        }
  
    }); //end-polymer
  })(); //end-function  &lt;/script &gt;
  </code></pre>
  
  <p>I am not able to see any data in my table. Is there anything wrong in defining the data source ? I am new to polymer so not sure if this is the right way to achieve it. </p>