title: Headers not being sent by iron-ajax
body: <p>I'm new to Polymer (I'm trying to learn how to also use it with Angular 2 as well concurrently) and after looking through the docs, it looks like this</p>
  
  <pre><code>&lt;iron-ajax url="https://httpbin.org/get" last-response="{{data}}" handle-as="json"
         headers="{'Authorization': 'pew'}" auto&gt;
  </code></pre>
  
  <p></p>
  
  <p>but the headers didn't contain the Authorization header</p>
  
  <pre><code>"headers": {
      "Accept": "application/json", 
      "Accept-Encoding": "gzip, deflate, sdch, br", 
      "Accept-Language": "en-US,en;q=0.8", 
      "Host": "httpbin.org", 
      "Origin": "http://localhost:3000", 
      "Referer": "http://localhost:3000/", 
      "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36"
  }
  </code></pre>
  
  <p>so after some Googling, I found this <a href="https://stackoverflow.com/a/34340468/2038087" title="SO question">SO question</a> and tried to do what was explained and my <code>iron-ajax</code> changed to </p>
  
  <pre><code>&lt;iron-ajax url="https://httpbin.org/get" last-response="{{data}}" handle-as="json"
         headers="'{{getHeadersURL()}}'" auto&gt;
  </code></pre>
  
  <p></p>
  
  <p>with <code>getHeadersURL()</code> looking like </p>
  
  <pre><code>var headers = {'Authorization': 'pew'};
  
  function getHeadersURL() {
      return headers;
  }
  </code></pre>
  
  <p>However, even after sending this, the headers are still the same as above. Is there something I'm not understanding or something I'm doing wrong.</p>