fix: call #dup would raise MissingAttributeError while putting :mount_on in options

[marsz]

If use :mount_on option and call #dup, it would raise ActiveModel::MissingAttributeError

class Attachment < ::ApplicationRecord
  mount_uploader(:file, FileUploader, mount_on: :stored_filename)
end

attachment.dup

Got error:

ActiveModel::MissingAttributeError:
       can't write unknown attribute `file`

[mshibuya]

Looks good, thank you!