fix empty params after whitelisting

lib/api/v3/relations/relations_api.rb

@@ -51,7 +51,7 @@ module API
           end
           post do
             authorize(:manage_work_package_relations, context: @work_package.project)
-            declared_params = declared(params).reject { |key, value| key.to_sym == :id || value.nil? }
+            declared_params = declared(params).with_indifferent_access.reject { |key, value| key == :id || value.nil? }
 
             relation = @work_package.new_relation.tap do |r|
               r.to = WorkPackage.visible.find_by id: declared_params['to_id']