HasOne Association#embedded_keys should return Symbol.

While debugging #414 I realize that `Association::HasOne#embedded_key` returns String while `Association::HasMany#embedded_key` returns Symbol due to [`pluralize` being called on the HasOne](https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializer/associations.rb#L53). I think they should be made consistent (symbol?).