code: public CorsConfigBuilder allowedRequestHeaders(final CharSequence... headers) {
          for (CharSequence header: headers) {
              requestHeaders.add(header.toString());
          }
          return this;
      }
docs: Specifies the if headers that should be returned in the CORS 'Access-Control-Allow-Headers'
  response header.
  
  If a client specifies headers on the request, for example by calling:
  <pre>
  xhr.setRequestHeader('My-Custom-Header', "SomeValue");
  </pre>
  the server will receive the above header name in the 'Access-Control-Request-Headers' of the
  preflight request. The server will then decide if it allows this header to be sent for the
  real request (remember that a preflight is not the real request but a request asking the server
  if it allow a request).
  
  @param headers the headers to be added to the preflight 'Access-Control-Allow-Headers' response header.
  @return {@link CorsConfigBuilder} to support method chaining.