Make gem work with google

Following the work and comments of @nbibler this fixes Issue [#7](https://github.com/jjbohn/omniauth-openid-connect/issues/7) when you use the following options:

* send_nonce: false
* client_auth_method: :not_basic

I'm not sure whether or not it's a good idea have #fix_user_info in there since it is very specific to google. Alternatively we could introduce another option to update user_info.