title: Polymer 2 iron-ajax with handleAs=xml bug?
body: <p>I have the following code:</p>
  
  <pre><code>&lt;iron-ajax verbose="true" auto method="GET" url="https://localhost/xformupload" handleAs="xml" headers='{"accept" : "text/xml", "mandator" : "xxx", "user" : "xxx", "password" : "xxx"}' params='{"command" : "list"}' on-response="_on_response" last-response="{{response}}"/&gt;
  </code></pre>
  
  <p>Reviewing this within Chrome debugger the request will be send correctly, also the answer (xml) is correctly. What drives me crazy here is that everything except the handleAs="xml" will be set correctly.</p>
  
  <p>But within the chrome debugger (iron-ajax.html:484) when the request is send - handleAs is always (damned) set as "json".</p>
  
  <p>Looks to me like polymer will not(!) set handleAs attribute correctly!</p>
  
  <p>Or did I miss something?</p>