Fix multi-value string data table

    <nuxeo-data-table items="{{document.properties.contract:contact}}" orderable="true" editable="true" role="widget" size="0">

      <nuxeo-data-table-column name="Name">
        <template>
          [[item.Name]]
        </template>
      </nuxeo-data-table-column>

      <nuxeo-data-table-column name="Number">
        <template>
          [[item.number]]
        </template>
      </nuxeo-data-table-column>

      <nuxeo-data-table-column name="Active">
        <template>
          [[item.active]]
        </template>
      </nuxeo-data-table-column>

      <nuxeo-data-table-form>
        <template>
          <nuxeo-input value="{{item.Name}}" label="Name" type="text"></nuxeo-input>
          <nuxeo-input value="{{item.number}}" label="Number" type="number"></nuxeo-input>
          <paper-checkbox checked="{{item.active}}"></paper-checkbox>
        </template>
      </nuxeo-data-table-form>
    </nuxeo-data-table>

    <nuxeo-data-table role="widget" items="{{task.variables.assignees}}" orderable="true" editable="true" size="0" scroll-throttle="60">
      <nuxeo-data-table-column name="Assignees">
        <template>
          [[item]]
        </template>
      </nuxeo-data-table-column>

      <nuxeo-data-table-form>
        <template>
          <nuxeo-input value="{{item}}" label="Assignees" type="text"></nuxeo-input>
        </template>
      </nuxeo-data-table-form>
    </nuxeo-data-table>