Add default value of `[]` for collection associations.

Starting all editions with empty arrays helps avoid
"you can't change this" issues during state changes
where the fields have remained empty but are no
longer null.

Switching to #changes rather than #changed_attributes
as it takes default values into account.
Add a debugger.
Merge pull request #232 from alphagov/add-default-values-for-collections

Add default values for collections