Fix #dup with custom serialisation

[nassim-platogo]

While write_uploader is aliased by default to write_attribute, it may be overridden in the model (see example below).

class SomeModel < ActiveRecord::Base
  store :settings, accessors: [:logo] # virtual column logo

  mount_uploader :logo, LogoUploader

  def write_uploader(column, identifier)
    settings[column.to_sym] = identifier
  end

  def read_uploader(column)
    settings[column.to_sym]
  end
end

However duplicating such a model fails with the following error.

[1] pry(main)> SomeModel.new.dup
ActiveModel::MissingAttributeError: can't write unknown attribute `logo

This change ensures that on duplication, the override write_uploader method is called when clearing the attribute.

[mshibuya]

You're right. Thanks!