code: public static <T> Consumer<T> spy(Consumer<T> consumer, Box<T> param) {
          return new CapturingConsumer<T>(consumer, param);
      }
docs: Proxies an consumer spying for parameter.
  
  @param <T> the consumer parameter type
  @param consumer the consumer to be spied
  @param param a box that will be containing the spied parameter
  @return the proxied consumer