title: How do I pass an attribute to a core-ajax in an element definition?
body: <p>I am creating an element, which takes one parameter: userid.
  Inside the component's template I have a core-ajax tag which needs to pass that attribute as a parameter.</p>
  
  <p>I am doing it like this:</p>
  
  <pre><code>&lt;template attributes="userid"&gt;
      &lt;core-ajax
          auto
          url="../../ajax/gettoday.php" 
          handleAs="json" 
          method="post" 
          params='{"userid": "{{userid}}"}'
          response="{{today}}"&gt;&lt;/core-ajax&gt;
          ...
  &lt;/template&gt;
  </code></pre>
  
  <p>Is it supposed to accept the attribute inside the params string?</p>