send_confirmation_instructions broken on queue_classic

Devise passes a `to` key as a symbol in the `send_confirmation_instructions` method:
https://github.com/plataformatec/devise/blob/master/lib/devise/models/confirmable.rb#L91

This breaks queue_classic which expects all keys to be string. This pull request calls stringify_keys! on the opts hash before passing it on to the configured worker.