Fix a race condition in the FilesPipeline

Checksum calculation could happen simultaniously with
persisting the file in the store (which is done in a thread):
they operated on the same buf object.
Concretely this lead to a bug with S3FilesStore
when using botocore: the signature did not match because
the position in the buf was already at the end.
The fix is to move checksum calculation before passing buf
to the store.

scrapy/pipelines/files.py

@@ -364,8 +364,9 @@ class FilesPipeline(MediaPipeline):
     def file_downloaded(self, response, request, info):
         path = self.file_path(request, response=response, info=info)
         buf = BytesIO(response.body)
-        self.store.persist_file(path, buf, info)
         checksum = md5sum(buf)
+        buf.seek(0)
+        self.store.persist_file(path, buf, info)
         return checksum
 
     def item_completed(self, results, item, info):