Fix HABTM query (#104)

While these seem like they should be equivalent, we are seeing the
preloaded data is not filtered by the FK.

Author: richmolj

lib/jsonapi_compliable/adapters/active_record_sideloading.rb

@@ -99,8 +99,7 @@ def has_and_belongs_to_many(association_name, scope: nil, resource:, foreign_key
               .class.reflections[through.to_s].klass.table_name
 
             _scope.call
-              .joins(through)
-              .preload(through) # otherwise n+1 as we reference in #assign
+              .includes(through)
               .where(table_name => { fk => parent_ids })
               .distinct
           end