[FIX] website: allow authenticate in json multidb

Before this commit launch a server with --db-filter that match at least 2 dbs name
Try to authenticate

You will have an error request is unbound when you try to access request.env

Now we retrieve the user from self instead of the request.

New test to ensure rpc authentication is tested.

Related to commit 245ef4b1

closes odoo/odoo#38047

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>