code: public <T> CorsConfigBuilder preflightResponseHeader(final CharSequence name, final Callable<T> valueGenerator) {
          preflightHeaders.put(name, valueGenerator);
          return this;
      }
docs: Returns HTTP response headers that should be added to a CORS preflight response.
  
  An intermediary like a load balancer might require that a CORS preflight request
  have certain headers set. This enables such headers to be added.
  
  Some values must be dynamically created when the HTTP response is created, for
  example the 'Date' response header. This can be accomplished by using a Callable
  which will have its 'call' method invoked when the HTTP response is created.
  
  @param name the name of the HTTP header.
  @param valueGenerator a Callable which will be invoked at HTTP response creation.
  @param <T> the type of the value that the Callable can return.
  @return {@link CorsConfigBuilder} to support method chaining.