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 
Regarding on repending on the old token_response value, if someone did that they were already in for a bad time. BOTH forms will appear in the serialized credential store, depending on whether or not a refresh has ever happened.