Flexible behaviour for STI models as Readable

[tangopium]

Allows STI subclasses to be defined as readable without affecting parent classes.

For example:

class Message < ActiveRecord::Base
    ...
end

class SystemMessage < Message
    acts_as_readable :on => :updated_at
end

class UserMessage < Message
    acts_as_readable :on => :updated_at
end

The modification allows to mark UserMessages all as read without doing the same (unintentional) for all Messages (including the SystemMessages)

[ledermann]

@tangopium: Great, thank you very much!