title: taking a screenshot with scrapy-selenium
body: <p>been trying to get scrapy-selenium to work for a while, I have the simple code from the tutorial and just trying to take a screenshot but for some reason I don't get a proper response, here is my code:</p>
  <pre><code>from shutil import which
  
  SELENIUM_DRIVER_NAME = 'firefox'
  SELENIUM_DRIVER_EXECUTABLE_PATH = which('geckodriver')
  SELENIUM_DRIVER_ARGUMENTS=['-headless']  # '--headless' if         
  using chrome instead of firefox
  
  DOWNLOADER_MIDDLEWARES = {
      'scrapy_selenium.SeleniumMiddleware': 800
  }
  
  from scrapy_selenium import SeleniumRequest
  
  
  def start():
      yield SeleniumRequest(
          url='www.google.com',
          callback=parse_result, wait_time=1000,
          screenshot=True
     )
  
  def parse_result(self, response):
      print(response.meta)
      with open('image.png', 'wb') as image_file:
          image_file.write(response.meta['screenshot'])
  
  
  start()
  </code></pre>
  <p>I am getting no screenshot key in response.meta..
  what am I missing ? by the way I am using python 3.10
  Thanks in advance :)</p>