Fix bug when calling Proceeding.check_by_fields.

modules/harvest_tools.py

@@ -1477,7 +1477,7 @@ class Proceedings(PublicationsTool):
         id, status = self.check_by_fields(authors=authors,
                                           conference_title=conference_title,
                                           first_author=first_author,
-                                          id_publisher=id_publisher,
+                                          id_publishers=id_publisher,
                                           oai_url=oai_url,
                                           preprint=preprint,
                                           pages=pages,