title: scrapy return empty json file
body: <p>I'm using scrapy in order to extract data from websites.
  When I'm opening the json result file it's always return empty.
  My scrapy code is attached:</p>
  
  <pre><code>from scrapy import Spider
  
  
  class StackSpider(Spider):
      name = "stack"
      allowed_domains = ["youtube.com"]
      start_urls = ["https://www.youtube.com/results?search_query=Motorcycle+Accident+Stunt+Rider+Knocks+Himself+Out+Stunt+Fail+2015"]
  
      def parse(self,response):
          questions = Selector(response).xpath('//a')
          for question in questions:
              item = StackItem()
              item['title'] = question.xpath(
                  'a/text()').extract()
              item['url'] = question.xpath('//@href]').extract()
              yield item
  </code></pre>