title: Polymer on-click is double posting
body: <p>I'm wondering if anybody here has had a similar problem with Polymer double posting a form.  My code is very simple, as stated previously, each time I click the button the form is posted twice. I'm using Polymer 05.5</p>
  
  <pre><code>&lt;polymer-element name="book-form"&gt;
      &lt;template&gt;
          &lt;label for="title"&gt;Title:&lt;/label&gt;
          &lt;input id="title" value="{{title}}" /&gt;
          &lt;label for="author"&gt;Author:&lt;/label&gt;
          &lt;input id="author" value="{{author}}" /&gt;
          &lt;label for="image"&gt;Image:&lt;/label&gt;
          &lt;input id="image" value="{{image}}" /&gt;
          &lt;button on-click="{{fireAjax}}"&gt;Submit Form&lt;/button&gt;
          &lt;core-ajax id="ajax"
                     auto
                     url="http://localhost:45922/api/book"
                     handleAs="json"
                     method="POST"
                     &gt;
          &lt;/core-ajax&gt;     
      &lt;/template&gt;
      &lt;script&gt;
          Polymer({
              fireAjax: function () {
                  var data = { image: this.image, author: this.author, title: this.title };
                  this.$.ajax.contentType = 'application/json';
                  this.$.ajax.body = JSON.stringify(data);
                  this.$.ajax.go();
              }
          });
      &lt;/script&gt;
  &lt;/polymer-element&gt;
  </code></pre>