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.