Use _read_attribute instead of []

It is not only faster, it also doesn't raise an exception if the column
was not fetched in the model using select.

Author: rafaelfranca

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -111,7 +111,7 @@ def require_counter_update?
         def replace_keys(record, force: false)
           target_key = record ? record._read_attribute(primary_key(record.class)) : nil
 
-          if force || owner[reflection.foreign_key] != target_key
+          if force || owner._read_attribute(reflection.foreign_key) != target_key
             owner[reflection.foreign_key] = target_key
           end
         end

activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb

@@ -19,7 +19,7 @@ def replace_keys(record, force: false)
 
           target_type = record ? record.class.polymorphic_name : nil
 
-          if force || owner[reflection.foreign_type] != target_type
+          if force || owner._read_attribute(reflection.foreign_type) != target_type
             owner[reflection.foreign_type] = target_type
           end
         end