OAuth2User should extend OAuth2AuthenticatedPrincipal

[jzheaux]

OAuth2AuthenticatedPrincipal is an AuthenticatedPrincipal that adds getAuthorities as well as getAttributes. It represents the principal associated with an OAuth 2.0 token.

A user is a type of principal, so having OAuth2User extend OAuth2AuthenticatedPrincipal makes sense.

Additionally, it will reduce the code that OAuth2User needs to maintain, plus it will get the nice benefit of a getAttribute default method.