title: FCM MissingAuthorization issue
body: <p>Here is my code:</p>
  
  <pre class="lang-js prettyprint-override"><code>function sendToken(token) {
    $.ajax({
      url: 'https://iid.googleapis.com/iid/v1/' + token + '/rel/topics/IoT',
      method: 'POST',
      headers: {
        Authorization: 'key=' + server_key
      },
      contentType: 'application/json',
      cache: false,
      success: function(response) {
        console.log(response);
      },
      error: function(response) {
        console.log(response);
      }
    });
  }
  </code></pre>
  
  <p>The response is <code>{"error":"MissingAuthorization"}</code></p>
  
  <p>I'm stuck and not able to understand why this is not working.</p>