Add Joseph Heenan to CONTRIBUTORS
Add Joseph Heenan to CONTRIBUTORS

For re-enabling "code id_token" as a partially supported response_type.

https://github.com/openid/AppAuth-iOS/pull/298

openid/AppAuth-iOS,openid/AppAuth-iOS,openid/AppAuth-iOS,WilliamDenniss/AppAuth-iOS,WilliamDenniss/AppAuth-iOS,WilliamDenniss/AppAuth-iOS