title: ElasticSearch Access-Control-Allow-Headers header is not present
body: <p>I am making a POST request from a local https server to an ElasticSearch endpoint which has been configured as follows</p>
  
  <pre><code>http.cors.enabled: true
  http.cors.allow-credentials: true
  http.cors.allow-origin: "*"
  http.cors.allow-methods: OPTIONS, HEAD, GET, POST, PUT, DELETE
  http.cors.allow-headers: X-Requested-With, X-Auth-Token, Content-Type, Content-Length, Authorization, Access-Control-Allow-Headers, Accept
  </code></pre>
  
  <p>The request has headers:</p>
  
  <pre><code>Access-Control-Allow-Headers: Accept, Access-Control-Allow-Headers, Authorization, Content-Type
  Content-Type: application/json; charset=utf-8
  Accept: application/json; charset=utf-8
  Access-Control-Allow-Credentials: true
  Authorization: (basic authentication token)
  </code></pre>
  
  <p>On a POST request, the following error appears: <code>Request header field Access-Control-Allow-Headers is not allowed by Access-Control-Allow-Headers in preflight response.</code></p>
  
  <p>The network debugger indeed shows that the <code>Access-Control-Allow-Headers</code> header is not present in the response header. The response header:</p>
  
  <pre><code>HTTP/1.1 200 OK
  Access-Control-Allow-Origin: https://dl.dropboxusercontent.com
  Vary: Origin
  Access-Control-Allow-Methods: 
  Access-Control-Allow-Credentials: true
  Access-Control-Max-Age: 1728000
  content-length: 0
  date: Fri, 29 Apr 2016 14:08:14 GMT
  </code></pre>
  
  <p>Note that <code>Access-Control-Allow-Headers</code> is not present and <code>Access-Control-Allow-Methods</code> is blank. All possible string formats have been tested, and these headers do not appear. </p>