Revert "Remove private verify readonly attr method"

This reverts commit 7a8aee08.

activerecord/lib/active_record/persistence.rb

@@ -232,7 +232,7 @@ def update_columns(attributes)
       raise ActiveRecordError, "can not update on a new record object" unless persisted?
 
       attributes.each_key do |key|
-        raise ActiveRecordError, "#{key} is marked as readonly" if self.class.readonly_attributes.include?(key.to_s)
+        raise ActiveRecordError, "#{key.to_s} is marked as readonly" if self.class.readonly_attributes.include?(key.to_s)
       end
 
       attributes.each do |k,v|
@@ -405,5 +405,9 @@ def create
       @new_record = false
       id
     end
+
+    def verify_readonly_attribute(name)
+      raise ActiveRecordError, "#{name} is marked as readonly" if self.class.readonly_attributes.include?(name)
+    end
   end
 end