* tls: Fall back to certificate keyed by empty name (fixes #2035)

This should only happen for sites defined with an empty hostname (like
":8080") and which are using self-signed certificates or some other
funky self-managed certificate. But that certificate should arguably
be used for all incoming SNI names.

* tls: Revert to serving any certificate if no match, regardless of SNI

Also fix self-signed certs to include IP addresses in their name
if they are configured to serve an IP address

* Remove tests which are now irrelevant (behavior reverted)

It would be good to revisit this in the future.