Fix pagination header error with marshmallow 2

Author: lafrech

flask_smorest/pagination.py

@@ -260,7 +260,10 @@ def _make_pagination_header(page, page_size, item_count):
                     page_header['previous_page'] = page - 1
                 if page < last_page:
                     page_header['next_page'] = page + 1
-        return PaginationHeaderSchema().dumps(page_header)
+        header = PaginationHeaderSchema().dumps(page_header)
+        if MARSHMALLOW_VERSION_MAJOR < 3:
+            header = header.data
+        return header
 
     @staticmethod
     def _prepare_pagination_doc(doc, doc_info, **kwargs):

tests/test_pagination.py

@@ -146,6 +146,8 @@ def func(pagination_parameters):
                 '{"total": 2, "total_pages": 1, '
                 '"first_page": 1, "last_page": 1, "page": 1}'
             )
+            # Also check there is only one pagination header
+            assert len(response.headers.getlist(header_name)) == 1
 
     def test_pagination_header_documentation(self, app):
         """Test pagination header is documented"""