cpp: Fix addrSpec for keys without email

* lang/cpp/src/key.cpp (UserID::addrSpec): Use uid->address instead
of normalizing again.
(&operator<<(std::ostream &, const UserID &): Print it.

This saves a normalization and fixes the case where a user id
is just a mail address without name, in that case gpgme sets
"address" but not email. Because the email is then the name.