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 only know this because a few days ago I was trying to figure out why something unrelated wasn't refreshing properly, and seeing the mismatched token_response in the before/after for the credential store was a bit of a red herring.