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 think i'm more just being wary about throwing it away on the off chance that people are using it -- though, as you point out, it could be that people are shooting themselves in the foot with it. ;)

let's go ahead and drop it for now, and i'll add a note in the `CHANGELOG` (so that someone who was looking for the old value can pipe up if it breaks them).