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.
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.
