`stored_attributes` need to be specific to a subclass.

Currently they are all stored globally in the same `Hash`.
This commit forces the creation of a per-class variable if necessary.

The behavior was exposed through the following test-case:

```
  1) Failure:
StoreTest#test_all_stored_attributes_are_returned [/Users/senny/Projects/rails/activerecord/test/cases/store_test.rb:151]:
--- expected
+++ actual
@@ -1 +1 @@
-[:color, :homepage, :favorite_food]
+[:resolution, :color, :homepage, :favorite_food]
```

1 files changed, 12 insertions, 0 deletions

diff --git a/activerecord/test/cases/store_test.rb b/activerecord/test/cases/store_test.rb
index c2c56abacd..0c9f7ccd55 100644
--- a/activerecord/test/cases/store_test.rb
+++ b/activerecord/test/cases/store_test.rb
@@ -150,4 +150,16 @@ class StoreTest < ActiveRecord::TestCase
   test "all stored attributes are returned" do
     assert_equal [:color, :homepage, :favorite_food], Admin::User.stored_attributes[:settings]
   end
+
+  test "stored_attributes are tracked per class" do
+    first_model = Class.new(ActiveRecord::Base) do
+      store_accessor :data, :color
+    end
+    second_model = Class.new(ActiveRecord::Base) do
+      store_accessor :data, :width, :height
+    end
+
+    assert_equal [:color], first_model.stored_attributes[:data]
+    assert_equal [:width, :height], second_model.stored_attributes[:data]
+  end
 end