Fix infinite loop when using nuxeo-document-attachments on singlevalued property

<!--
`nuxeo-bim-view-layout`
@group Nuxeo UI
@element nuxeo-bim-view-layout
-->
<dom-module id="nuxeo-bim-view-layout">
  <template>
    <style>
      *[role=widget] {
        padding: 5px;
      }
    </style>   

 <div role="widget">
      <label>Svblob view - nuxeo-document-attachments</label>
      <nuxeo-document-attachments document="{{document}}" xpath="esv:svblob"></nuxeo-document-attachments>
  </div>  

</template>  
<script>
  Polymer({
    is: 'nuxeo-bim-view-layout',
    behaviors: [Nuxeo.LayoutBehavior],
    properties: {      /**
         * @doctype bim
         */
      document: {
        type: Object,
      },    }
  });
  </script>
</dom-module>