[storages] Seek Binary before and after copying it

cubicweb_s3storage/storages.py

@@ -69,7 +69,9 @@ class S3Storage(Storage):
             # copy Binary value, workaround for boto3 bug
             # https://github.com/boto/s3transfer/issues/80
             # .read() is required since Binary can't wrap itself
+            binary.seek(0)
             buffer = Binary(binary.read())
+            binary.seek(0)
             self.debug('Upload object to S3')
             # upload_fileobj should make automagically a multipart upload if
             # needed