docs: public Spider setScheduler(Scheduler scheduler) {
          checkIfRunning();
          Scheduler oldScheduler = this.scheduler;
          this.scheduler = scheduler;
          if (oldScheduler != null) {
              Request request;
              while ((request = oldScheduler.poll(this)) != null) {
                  this.scheduler.push(request, this);
              }
          }
          return this;
      }