Multisite login only works on domain where key was registered

[iandunn]

1. Setup a sub-domain Multisite where the main site is example.org, and there's another site named login.example.org
2. Register a WebAuthn key at example.org/wp-admin/profile.php
3. Login to example.org/wp-login.php. Authentication will succeed.
4. Logout.
5. Login to login.example.org/wp-login.php. Authentication will fail.

I don't see where the rpId is being set, is it using a default value somewhere? If I understand correctly, setting that to example.org should allow authentication on login.example.org to succeed.

[sjinks]

Fixed in #461

[sjinks]

Landed to 2.1.0.