Don't expose internal methods in Preloader::ThroughAssociation

`through_reflection` and `source_reflection` are used only in the class.

activerecord/lib/active_record/associations/preloader/through_association.rb

@@ -4,14 +4,6 @@ module ActiveRecord
   module Associations
     class Preloader
       module ThroughAssociation #:nodoc:
-        def through_reflection
-          reflection.through_reflection
-        end
-
-        def source_reflection
-          reflection.source_reflection
-        end
-
         def run(preloader)
           already_loaded     = owners.first.association(through_reflection.name).loaded?
           through_scope      = through_scope()
@@ -48,6 +40,13 @@ def run(preloader)
         end
 
         private
+          def through_reflection
+            reflection.through_reflection
+          end
+
+          def source_reflection
+            reflection.source_reflection
+          end
 
           def preload_index
             @preload_index ||= @preloaded_records.each_with_object({}).with_index do |(id, result), index|