title: Polymer property not updating via data binding
body: <p>I have a Polymer 1.0 custom element that wraps a text box (an HTML input element). Here is the template:</p>
  
  <pre><code>&lt;template&gt;
      &lt;input type="text" value="{{value}}"&gt;
  &lt;/template&gt;
  </code></pre>
  
  <p>And here is the value property:</p>
  
  <pre><code> value: {
              type: String,
              value: '',
              notify: true,
              observer: '_valueChanged'
          }
  </code></pre>
  
  <p>As I type into the text box shouldn't the _valueChanged observer get invoked each time the input text is updated? The behaviour I'm encountering is that the observer only gets called once during initialisation of the custom element.</p>