title: Polymer use Array for data binding
body: <p>I have this dom-repeat template and I wonder how to pass an Array into attribute.</p>
  
  <pre><code>&lt;dom-module id="my-element"&gt;
    &lt;template&gt;
          &lt;template is="dom-repeat" items="{{headers}}"&gt;
            &lt;span&gt;{{item}}&lt;/span&gt;
          &lt;/template&gt;
    &lt;/template&gt;
  &lt;/dom-module&gt;
  
  &lt;script&gt;
    Polymer({
      is: "my-element",
      properties: {
        headers: Array
      }
    });
  &lt;/script&gt;
  </code></pre>
  
  <p>and then in an <code>index.html</code> file, I bind it like this:</p>
  
  <pre><code>&lt;my-element headers="['abc', 'def']"&gt;&lt;/my-element&gt;
  </code></pre>
  
  <p>I tried doing that but I nothing happened and I got the warning <code>Polymer::Attributes: couldn't decode Array as JSON</code></p>