Allow ActiveSupport::HashWithIndifferentAccess of yaml parse

[llpereiras]

The serialized field are stored with ActiveSupport

In my model, I added this:

  field :description, :serialized

when I read the record, I received this error:

Psych::DisallowedClass: Tried to load unspecified class: ActiveSupport::HashWithIndifferentAccess

I think, the most of the apps use rails and ActiveSupport, so I think should be a good addiction to add the support of ActiveSupport

For the extra informattion, I doesn't use active record gem.
Because of that, the configuration of active_record like this
config.active_record.use_yaml_unsafe_load
OR
config.active_record.yaml_column_permitted_classes = [ActiveSupport::HashWithIndifferentAccess]
doesn't work for me.

[andrykonchin]

I agree that it would be convenient to allow serialising such classes like HashWithIndifferentAccess. So the only missing thing in the PR is specs for the new feature. Could you please add them?

But TBH I like mentioned above config option yaml_column_permitted_classes. And use_yaml_unsafe_load could also be useful or at least convenient for developers that are familiar with ActiveRecord. So I am open to adding them.