[BUG] mid_registrar wrong expiry value

[vasilevalex]

OpenSIPS version you are running

version: opensips 3.2.3 (x86_64/linux)
flags: STATS: On, DISABLE_NAGLE, USE_MCAST, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, HP_MALLOC, DBG_MALLOC, FAST_LOCK-ADAPTIVE_WAIT
ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144, MAX_LISTEN 16, MAX_URI_SIZE 1024, BUF_SIZE 65535
poll method support: poll, epoll, sigio_rt, select.
git revision: f32089b8c
main.c compiled on  with gcc 8

Describe the bug
During unregister, "expires" parameter in Contact header is wrong.

To Reproduce
Steps to reproduce the behavior:
Example:

1. Start OpenSIPS
2. Use mid_registrar in mode "1"
3. Check REGISTER message sent to registrar when UA tries to un-register

Expected behavior
Expires parameter in Contact header must be "0".

Relevant System Logs

Nov 29 14:52:46 midregistrar1 /usr/sbin/opensips[26329]: DBG:mid_registrar:calc_ob_contact_expires: outgoing expires: 0
Nov 29 14:52:46 midregistrar1 /usr/sbin/opensips[26329]: DBG:mid_registrar:save_restore_rpl_contacts:     >> REGISTER -1638193966s ------- -1638193966s 200 OK <<!
Nov 29 14:52:46 midregistrar1 /usr/sbin/opensips[26329]: DBG:usrloc:get_ucontact: using ct matching mode 0

OS/environment information

• Operating System: Debian 10.10
• OpenSIPS installation: git

Additional context