Do not convert attrs to String until needed

lib/active_model/serializer.rb

@@ -64,8 +64,6 @@ module ActiveModel
         options = attrs.extract_options!
 
         attrs.each do |attr|
-          attr = attr.to_s
-
           unless method_defined?(attr)
             define_method attr do
               object.send attr

lib/active_model/serializer/associations.rb

@@ -5,7 +5,7 @@ module ActiveModel
   class Serializer
     class Association
       def initialize(name, options={})
-        @name          = name
+        @name          = name.to_s
         @options       = options
 
         self.embed     = options[:embed] || SETTINGS[:embed]