`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, 3 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/store.rb b/activerecord/lib/active_record/store.rb
index a610f479f2..5fc80cdb6d 100644
--- a/activerecord/lib/active_record/store.rb
+++ b/activerecord/lib/active_record/store.rb
@@ -86,6 +86,9 @@ module ActiveRecord
           end
         end
 
+        # assign new store attribute and create new hash to ensure that each class in the hierarchy
+        # has its own hash of stored attributes.
+        self.stored_attributes = {} if self.stored_attributes.blank?
         self.stored_attributes[store_attribute] ||= []
         self.stored_attributes[store_attribute] |= keys
       end