title: paper-dropdown-menu default selected option doesn't work
body: <p>My Expectation here would be that 'Option 1' would be selected by default once the page loads. This is not what is happening. Instead it shows the label, not the selected option based on the paper-item value attribute.</p>
  
  <pre><code>&lt;paper-dropdown-menu name="dropdown" label="Dropdown" selected='1' valueattr='value'&gt;
      &lt;paper-dropdown class="dropdown"&gt;
          &lt;core-menu class="menu"&gt;
              &lt;paper-item value="1"&gt;Option 1&lt;/paper-item&gt;
              &lt;paper-item value="2"&gt;Option 2&lt;/paper-item&gt;
          &lt;/core-menu&gt;
      &lt;/paper-dropdown&gt;
  &lt;/paper-dropdown-menu&gt;
  </code></pre>