Urls will be lost when call setScheduler()

[code4craft]

When call Spider.setScheduler(), the urls in old scheduler will be lost. Try to fix it by polling all urls from old scheduler and pushing to the new one.

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