fix and optimice authentication_keys

This is a Fix for [#3218](https://github.com/plataformatec/devise/pull/3218#discussion_r17965071).

And now it use the I18n key support.array.words_connector to connect the authentication keys.
No!

The I18n message is: [`"Invalid %{authentication_keys} or password."`](https://github.com/plataformatec/devise/blob/master/config/locales/en.yml#L12)
The usage of `to_sentence` will result in `Invalid a, b and c or password.`.