(fix) ensure aggregate fields are loaded in Graphql 2.3 (#18)

* fix: initialize attributes array

* fix: ensure loaded

lib/graphql/groups/has_aggregates.rb

@@ -31,6 +31,7 @@ def aggregate(name, *_, **options, &block)
 
         def aggregate_field(*args, **kwargs, &block)
           field_defn = Schema::AggregateField.from_options(*args, owner: self, **kwargs, &block)
+          field_defn.ensure_loaded
           add_field(field_defn)
           field_defn
         end