[Brad Bennett, Jessica Yao, & Lakshmi Parthasarathy]

Renaming the test accordingly to its behaviour

Adding 'Fixes' statement to changelog

Improving tests legibility & changelog

Undoing mistakenly removed empty line & further improving changelog

refs #11561 #12700

The use of `store` on Hstore fields (for instance) is not needed
because serialization aimed to interact with Ruby objects and not
with provided field types

This is necessary because as of 5ac2341f `hstore` columns are always stored
as `Hash` with `String` keys. `ActiveRecord::Store` expected the attribute to
be an instance of `HashWithIndifferentAccess`, which led to the bug.

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]
```

Allow store accessors to be overrided like other attribute methods,
e.g.:

    class User < ActiveRecord::Base
      store :settings, accessors: [ :color, :homepage ], coder: JSON

      def color
        super || 'red'
      end
    end

This allows you to retrieve the list of attributes you've defined.
Usable for e.g. selects in the view, or interators based on the
attributes you wish to store in the serialized column.

Fix backward compatibility with stored Hash values. Wrap coders to convert serialized values to indifferent access.

In order to make migration from 3.x apps easier, we should try to
convert
Hash instances to HashWithIndifferentAccess, to allow accessing values
with both symbol and a string. This is follow up to changes in 3c0bf043.