AutoThrottle: respect download_delay=0 spider attribute

Author: kmike

scrapy/extensions/throttle.py

@@ -28,7 +28,7 @@ def _spider_opened(self, spider):
 
     def _min_delay(self, spider):
         s = self.crawler.settings
-        return getattr(spider, 'download_delay', 0.0) or s.getfloat('DOWNLOAD_DELAY')
+        return getattr(spider, 'download_delay', s.getfloat('DOWNLOAD_DELAY'))
 
     def _max_delay(self, spider):
         return self.crawler.settings.getfloat('AUTOTHROTTLE_MAX_DELAY')