Revert "Revert "More 1.9 way""

Fixed failing tests
This reverts commit 8350ce97f2.

actionpack/test/template/form_helper_test.rb

@@ -867,6 +867,7 @@ class FormHelperTest < ActionView::TestCase
 
   def test_form_for_with_remote_without_html
     @post.persisted = false
+    def @post.id; nil; end
     form_for(@post, :remote => true) do |f|
       concat f.text_field(:title)
       concat f.text_area(:body)
@@ -1016,6 +1017,7 @@ class FormHelperTest < ActionView::TestCase
     old_locale, I18n.locale = I18n.locale, :submit
 
     @post.persisted = false
+    def @post.id; nil; end
     form_for(@post) do |f|
       concat f.submit
     end

activemodel/lib/active_model/conversion.rb

@@ -39,11 +39,9 @@ module ActiveModel
 
     # Returns an Enumerable of all key attributes if any is set, regardless
     # if the object is persisted or not.
-    #
-    # Note the default implementation uses persisted? just because all objects
-    # in Ruby 1.8.x responds to <tt>:id</tt>.
     def to_key
-      persisted? ? [id] : nil
+      key = respond_to?(:id) && id
+      key ? [key] : nil
     end
 
     # Returns a string representing the object's key suitable for use in URLs,