code: public void setExposeHeaders(String... header) {
          exposeHeadersSet.addAll(Arrays.asList(header));
          exposeHeaders = Joiner.on(",").join(header);
      }
docs: During a CORS request, the browser can only access simple response headers such as:
  
  <ul>
  <li>Cache-Control</li>
  <li>Content-Language</li>
  <li>Content-Type</li>
  <li>Expires</li>
  <li>Last-Modified</li>
  <li>Pragma</li>
  </ul>
  
  If you want clients to be able to access other headers, you have to Expose these headers.
  @param header