Simplify serializable_hash implementation

Now that Rails 3.1 is not supported anymore, we don't need to implement
to_xml, since it does the right thing by calling serializable_hash.

This removes the class_eval need that existed to simplify the
implementation of both to_xml and serializable_hash.

README.md

@@ -516,7 +516,7 @@ Devise supports ActiveRecord (default) and Mongoid. To select another ORM, simpl
 
 ### Heroku
 
-Using Devise on Heroku with Ruby on Rails 3.1 requires setting:
+Using Devise on Heroku with Ruby on Rails 3.2 requires setting:
 
 ```ruby
 config.assets.initialize_on_precompile = false

lib/devise/models/authenticatable.rb

@@ -96,18 +96,12 @@ module Devise
       def authenticatable_salt
       end
 
-      array = %w(serializable_hash)
-      # to_xml does not call serializable_hash on 3.1
-      array << "to_xml" if ActiveModel::VERSION::STRING[0,3] == "3.1"
-
-      array.each do |method|
-        class_eval <<-RUBY, __FILE__, __LINE__
-          # Redefine to_xml and serializable_hash in models for more secure defaults.
+      # Redefine serializable_hash in models for more secure defaults.
       # By default, it removes from the serializable model all attributes that
       # are *not* accessible. You can remove this default by using :force_except
       # and passing a new list of attributes you want to exempt. All attributes
       # given to :except will simply add names to exempt to Devise internal list.
-          def #{method}(options=nil)
+      def serializable_hash(options = nil)
         options ||= {}
         options[:except] = Array(options[:except])
 
@@ -116,10 +110,9 @@ module Devise
         else
           options[:except].concat BLACKLIST_FOR_SERIALIZATION
         end
+
         super(options)
       end
-        RUBY
-      end
 
       protected