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 
Yes, I can do that, it will avoid any problems if someone uses the d['id_token'] as the extracted_id_token.
Do you want I submit an other Pull request?