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 agree with you John, and in my initial fix I didn't modify the "token_response" structure, I've just let the token id in its original format, because in the current code, the token id of the "token_response" is overwritten.
And like I said in the Pull request comment, there is an utility, the OAuth un-extracted token id is needed for the Amazon AWS Web identity federation.
http://docs.aws.amazon.com/STS/latest/UsingSTS/web-identity-federation.html
Maybe there is a better way to do that?