Indeterminate behaviour when editing a member that is a "default admin"

[NightJar]

Like Schrödinger's cat, default admin is both a user, and not a user - in that the account exists always, but only while being observed. Editing this magically inserted default record causes strange behaviour.

Recreate:

1. Install SilverStripe using SS_DEFAULT_ADMIN_... combo of admin:password
2. Log in to /admin/security using admin:password
3. Open only user (admin)
4. Change "Default Admin" email address

   - admin
   + admin@localhost
   

5. save ("Apply changes").
6. wait for session to time out, perhaps whilst you eat lunch.
7. Click save again.
8. When presented with the "please re-authenticate that you are you" modal, enter the password (password)
9. Be told by the modal reload that your user does not exist, you are a fraud, and watch your whole DEFAULT_ADMIN life crumble before your eyes as if it were all an elaborate fabrication.
10. visit /admin/security and try to log in as admin@localhost:password - and be denied also.
11. visit /admin/security and log in as normal; using admin:password still.
12. Now there are two "Default Admin" users - admin and admin@localhost