title: Post json object with iron-ajax as application/x-www-form-urlencoded
body: <p>in my Polymer-based application I would like to use the fancy JavaScript model binding to input elements.</p>
  
  <p>The POST of the model to my Spring controller however I would like to perform as classic formdata, so that I can take advantage of Spring SessionAttributes.</p>
  
  <p>I set up <code>iron-ajax</code> like this:</p>
  
  <pre><code>&lt;iron-ajax id="saveMailing"
      method="POST"
      url="/api/mailing"
      content-type="application/x-www-form-urlencoded"
      on-response="mailingSaved"&gt;
  &lt;/iron-ajax&gt;
  </code></pre>
  
  <p>I have a JavaScript method which on button click executes the request:</p>
  
  <pre><code>saveDraft: function() {
      this.$.saveMailing.body = this.mailing;
      this.$.saveMailing.generateRequest();
  }
  </code></pre>
  
  <p><code>this.mailing</code> is a JSON-object. It's <em>not</em> FormData().</p>
  
  <p>The result is that I get a JavaScript error "Cannot read property toString of null", in iron-request.html:421</p>
  
  <p>Is it possible at all to do what I want ?  I mean passing <code>iron-ajax</code> a JSON-Object and expecting it to transform it to FormData. Or do I not use <code>iron-ajax</code> correctly ?</p>