Warn when performing certificate verification if server-identity is NULL
Warn when performing certificate verification if server-identity is NULL

It doesn't make sense for client connections to perform certificate
verification if server-identity is NULL. The only reason this would ever
not fail would be if the application is ignoring identity mismatches,
which is always insecure. So if you're doing that, you deserve a
warning. More likely, the programmer is just confused, and a warning
will help.

Failure to set server-identity stumped me for over an hour recently.
Having a warning would have made the problem immediately obvious.

Fixes #130

GNOME/glib-networking,GNOME/glib-networking,GNOME/glib-networking