fixed require for google_oauth2 strategy

[cbuggle]

When including google_oauth2 strategy i ran into the error that the strategy could not be found.

Fixed that.

[josevalim]

I don't think we should list all strategies. Maybe we should just try to require that file and rescue LoadError in case it doesn't work?

[rafaelfranca]

This is a better solution. What do you think @buggle? Could you provide a new pull request changing the way that we require the strategies?

[cbuggle]

sorry, quite busy at work. that's also why my pull request message was so brief.

After looking into it, i suppose it's caused by the mismatch of name "google-oauth2" and file "google_oauth2" of that strategy. Maybe we should just fix it right there...

Yetfor now i think this is a feasible solution, since we assure all major providers ( fb, twitter, google) work out of the box, and it is not that i introduced a whole new condition in that case statement just for google ...