res_pjsip_outbound_registration: only update the Expires header if the value has...

res_pjsip_outbound_registration: only update the Expires header if the value has changed

The PJSIP outbound registration API has undocumented behavior when reconfiguring
the outbound registration if the expires value being set is the same as what was
previously set.

In this case PJSIP will remove the Expires header entirely from subsequent
outbound REGISTER requests. To eliminate this as an issue we now check the current
expires value against the configured expires value and only apply it if it differs.

This ensures that outbound REGISTER requests always contain an Expires header.

Resolves: #1859