call constants on self to make extending the class easier

I've made a minor change that makes extending the strategy easier.

I'm working on an app where we wanted to split up Google/YouTube authentication and tried solving that by creating strategies like:

```
class OmniAuth::Strategies::Youtube < OmniAuth::Strategies::GoogleOauth2
  DEFAULT_SCOPE = 'userinfo.email,youtube'
  option :name, 'youtube'
end
```

This doesn't work though, when going through the `authorize_params` method the strategy used GoogleOauth2's DEFAULT_SCOPE. This PR fixes that problem (while keeping the old behaviour intact for other users).

I've added a spec that proves the change is working, however I could use some help cleaning up the spec. There should be an easier (and more readable) way to prove that the change is working.