GTlsClientConnection warning about NULL server-identity property

Thanks for the release of balsa 2.6.0!

I tested it out today with a connection to imap://bob@protected-headers.cmrg.net, and found this warning on stderr:

(balsa:891359): GLib-Net-WARNING **: 12:43:11.244: GTlsClientConnection certificate verification will fail because its server-identity property is NULL. Fix your application!

The application worked as i expect it to, but i haven't tested it against (for example) an invalid x.509 certificate.

I haven't dug into it further yet, sorry! If balsa is doing its own certificate checking, then it should probably also do something to suppress this warning so that people don't freak out about it.