Fix auth code argument to saveToken()

[jhermsmeier]

Fixes #557

[jcdogo]

This is a bad fix and should be closed. It actually breaks the server. code.authorizationCode is used in 5 different files, whereas this only changes it in one file. In addition, even if you changed all 5 files, it would become a breaking change for all existing users of this library.

Rather than changing the code, the model spec just needs to be correct to reflect what the code is actually doing.

[jhermsmeier]

Thanks for that valuable, and friendly comment. I'm just going to leave it to you to do the good fixes then :)