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

res/res_pjsip_outbound_registration.c

@@ -2069,6 +2069,7 @@ static int sip_outbound_registration_perform(void *data)
 	struct sip_outbound_registration *registration = ao2_bump(state->registration);
 	size_t i;
 	int max_delay;
+	pjsip_regc_info info;
 
 	/* Just in case the client state is being reused for this registration, free the auth information */
 	ast_sip_auth_vector_destroy(&state->client_state->outbound_auths);
@@ -2096,7 +2097,14 @@ static int sip_outbound_registration_perform(void *data)
 	state->client_state->auth_rejection_permanent = registration->auth_rejection_permanent;
 	max_delay = registration->max_random_initial_delay;
 
-	pjsip_regc_update_expires(state->client_state->client, registration->expiration);
+	/*
+	 * pjsip_regc_update_expires will remove the Expires header from the REGISTER request if the
+	 * expiration interval is re-set to the same value as the current interval.  We want to avoid
+	 * this so we only call it if the interval has changed.
+	 */
+	if (pjsip_regc_get_info(state->client_state->client, &info) == PJ_SUCCESS && info.interval != (unsigned) registration->expiration) {
+		pjsip_regc_update_expires(state->client_state->client, registration->expiration);
+	}
 
 	/* n mod 0 is undefined, so don't let that happen */
 	schedule_registration(state->client_state, (max_delay ? ast_random() % max_delay : 0) + 1);