libcli/smb: fix non mendatory signing against some vendor SMB2 servers.

libcli/smb: fix non mendatory signing against some vendor SMB2 servers.

Windows and Samba always sign the final session setup response
even if signing is not mendatory, but it ensures that the signing
key is correctly in place.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10146

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Tue Sep 17 09:40:10 CEST 2013 on sn-devel-104