Fix compatibility with JBuilder 2.11.3

Turn page_types into array since JBuilder
2.11.3 (rails/jbuilder#501) requires
collection to respond to `empty?`, which `Enumerable` does not
provide. See also rails/jbuilder#514

REDMINE-19357

Author: tf

app/helpers/pageflow/page_types_helper.rb

@@ -4,7 +4,7 @@ module PageTypesHelper
 
     def page_type_json_seeds(config)
       render_json_partial('pageflow/page_types/page_type',
-                          collection: config.page_types,
+                          collection: config.page_types.to_a,
                           as: :page_type)
     end