gnutls: fix g_tls_connection_gnutls_close_async to allow NULL cancellable
gnutls: fix g_tls_connection_gnutls_close_async to allow NULL cancellable

https://bugzilla.gnome.org/show_bug.cgi?id=659786

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