title: Polymer 1.0.5: Select not working on paper-dropdown-menu
body: <p>I can't seem to set a default selected option, if needed. Read the <a href="https://elements.polymer-project.org/elements/paper-dropdown-menu" rel="nofollow">docs</a> but no other properties, except for label works.</p>
  
  <pre><code>  &lt;paper-dropdown-menu label="I Love:"&gt;
      &lt;paper-menu class="dropdown-content" selectedItem="Heroes Reborn"&gt;
        &lt;paper-item&gt;Teen Wolf&lt;/paper-item&gt;
        &lt;paper-item&gt;Heroes Reborn&lt;/paper-item&gt;
        &lt;paper-item&gt;Once Upon a Time&lt;/paper-item&gt;
      &lt;/paper-menu&gt;
    &lt;/paper-dropdown-menu&gt;
  </code></pre>
  
  <p>No matter if I set a value on <code>paper-item value="x"</code> then <code>&lt;paper-menu selected="x"&gt;</code> nothing works. Do I need to run some other script?</p>