`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
Merge pull request #14833 from jyao6/attribute_inheritance

Fixed Attribute Inheritance Issue

Conflicts:
	activerecord/CHANGELOG.md