OAuth2Authenticator is considered deprecated, and isn't used in core. However, some plugins still depend on it, and this was breaking the signup of previously-staged users. There is no easy way to make an end-end test of this in core, but I will be adding an integration test in the SAML plugin.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
