Hello Is there any reason why callback_url is overriden [line 37](https://github.com/intridea/omniauth-oauth2/blob/master/lib/omniauth/strategies/oauth2.rb#L37)? It's losing query_string from its [original definition](https://github.com/intridea/omniauth/blob/master/lib/omniauth/strategy.rb#L414), which I find very useful... Cheers Ben