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 
this seems wise, but now i wonder: is anyone already depending on getting the extracted token back in the `token_response` field below?

maybe add it into the response as `d['extracted_id_token']`?