code: public <I extends HttpRequest, O extends HttpResponse>
      B decorator(DecoratingClientFunction<I, O> decorator) {
          decoration.add(decorator);
          return self();
      }
docs: Adds the specified HTTP-level {@code decorator}.
  
  @param decorator the {@link DecoratingClientFunction} that intercepts an invocation
  @param <I> the {@link Request} type of the {@link Client} being decorated
  @param <O> the {@link Response} type of the {@link Client} being decorated