[FEATURE] 'has_uuids' helper add uuid by default just after initialization.

Instead of adding a default uuid before record creation, it is added just after its initialization.

  class MyModel < ActiveRecord::Base
    include Uuids::Base
    has_uuids
  end

  item = MyModel.new
  item.uuid.present? # => true

== Changes

* Uuids::Base.has_uuids
* Uuids::Uuid - default value assigned after initialization.

== Version

Changed to 2.0.0 (backward incompatible).
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