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]
```

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/cases/store_test.rb

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Fix bug where `ActiveRecord::Store` used a global `Hash` to keep track of
+    all registered `stored_attributes`. Now every subclass of
+    `ActiveRecord::Base` has it's own `Hash`.
+
+    *Yves Senn*
+
 *   `change_column_default` allows `[]` as argument to `change_column_default`.
 
     Fixes #11586.

activerecord/lib/active_record/store.rb

@@ -86,6 +86,9 @@ def store_accessor(store_attribute, *keys)
           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

activerecord/test/cases/store_test.rb

@@ -183,4 +183,16 @@ class StoreTest < ActiveRecord::TestCase
     assert_equal dumped, second_dump
     assert_equal @john, YAML.load(second_dump)
   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