Fixed notification of selectionValue array items

[bendavis78]

The following use-case was not working, as notifications were not being sent for selectedValues items

      <div>
        <h4>Multi-select</h4>
        <div class="horizontal-section">
          <paper-menu id="menu" multi selected-values="{{selected}}">
            <paper-item>Bold</paper-item>
            <paper-item>Italic</paper-item>
            <paper-item>Underline</paper-item>
            <paper-item>Strikethrough</paper-item>
          </paper-menu>
        </div>
      </div>
      <div>
        <h4>Selected items:</h4>
        <div class="horizontal-section">
          <ul>
            <template is="dom-repeat" items="{{selected}}">
              <li>{{item}}</li>
            </template>
          </ul>
        </div>
      </div>

[bicknellr]

Fixed by #68.