Patch to keep the raw value of the OAuth2 id_token

Path to keep the raw value of the id_token in the token_reponse attribute of the OAuth2Credentials object.
This value is needed in some case, for example for the Amazon AWS Web identity federation.
See issue #64 
I'm saying that a field named "token_response" should always have the same structure. having some extra info there doesn't bother me, but it seems like limited utility and we might be able to safely omit it.