move buffer pointing to start of file before computing checksum. refs scrapy#92

Author: dangra

scrapy/contrib/pipeline/images.py

@@ -252,6 +252,7 @@ def image_downloaded(self, response, request, info):
         checksum = None
         for key, image, buf in self.get_images(response, request, info):
             if checksum is None:
+                buf.seek(0)
                 checksum = md5sum(buf)
             self.store.persist_image(key, image, buf, info)
         return checksum