`values[:includes]` in `reflection_scope` is not compatible with `through_scope`

Without this fix, preloading `:comments_with_include` will cause the
following error:

```
% bundle exec ruby -w -Itest test/cases/associations/eager_test.rb -n test_eager_with_has_many_through_join_model_with_include
Using sqlite3
Run options: -n test_eager_with_has_many_through_join_model_with_include --seed 1502

E

Error:
EagerAssociationTest#test_eager_with_has_many_through_join_model_with_include:
ActiveRecord::AssociationNotFoundError: Association named 'post' was not found on Post; perhaps you misspelled it?
```

1 files changed, 6 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index 874b3fbe85..b469a9ca10 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -86,9 +86,14 @@ module ActiveRecord
             if options[:source_type]
               scope.where! reflection.foreign_type => options[:source_type]
             elsif !reflection_scope.where_clause.empty?
-              scope.includes_values = Array(values[:includes] || options[:source])
               scope.where_clause = reflection_scope.where_clause
 
+              if includes = values[:includes]
+                scope.includes!(source_reflection.name => includes)
+              else
+                scope.includes!(source_reflection.name)
+              end
+
               if joins = values[:joins]
                 scope.joins!(source_reflection.name => joins)
               end