title: Scrapy CSV output duplicates fields
body: <p>I have a spider (below) and I want to be able to run it via a Cron job every 10 days or so, however, every time I run it after the first time. It rewrite the Fields instead of just appending the items to the appropriate fields in the CSV. How can I make it so that I only have one group of field headers at the top and all the data below it no matter how many times I run it?</p>
  
  <pre><code>import scrapy
  
  class Wotd(scrapy.Item):
      word = scrapy.Field()
      definition = scrapy.Field()
      sentence = scrapy.Field()
      translation = scrapy.Field()
  
  
  class WotdSpider(scrapy.Spider):
      name = 'wotd'
      allowed_domains = ['www.spanishdict.com/wordoftheday']
      start_urls = ['http://www.spanishdict.com/wordoftheday/']
      custom_settings = {
          #specifies exported fields and their order
      'FEED_EXPORT_FIELDS': ['word','definition','sentence','translation']
      }
  
  def parse(self, response):
      jobs = response.xpath('//div[@class="sd-wotd-text"]')
      for job in jobs:
          item = Wotd()
          item['word'] = job.xpath('.//a[@class="sd-wotd-headword-link"]/text()').extract_first()
          item['definition'] = job.xpath('.//div[@class="sd-wotd-translation"]/text()').extract_first()
          item['sentence'] = job.xpath('.//div[@class="sd-wotd-example-source"]/text()').extract_first()
          item['translation'] = job.xpath('.//div[@class="sd-wotd-example-translation"]/text()').extract_first()
          yield item 
  </code></pre>
  
  <p>From what I have been reading on the Scrapy docs it looks like I may have to mess around with the CsvItemExporter class and set include_headers_line=False but I am not sure where to add that class in the project structure.</p>