UaSettings: set sessionTimersRefreshMethod to string because constants are not exported

Author: eschmidbauer

lib/src/sip_ua_helper.dart

@@ -182,7 +182,7 @@ class SIPUAHelper extends EventManager {
     _settings.session_timers = uaSettings.sessionTimers;
     _settings.ice_gathering_timeout = uaSettings.iceGatheringTimeout;
     _settings.session_timers_refresh_method =
-        uaSettings.sessionTimersRefreshMethod;
+        uaSettings.sessionTimersRefreshMethodEnum;
     _settings.instance_id = uaSettings.instanceId;
     _settings.registrar_server = uaSettings.registrarServer;
     _settings.contact_uri = uaSettings.contact_uri != null
@@ -921,5 +921,14 @@ class UaSettings {
   /// Controls which kind of messages are to be sent to keep a SIP session
   /// alive.
   /// Defaults to "UPDATE"
-  DartSIP_C.SipMethod sessionTimersRefreshMethod = DartSIP_C.SipMethod.UPDATE;
+  String sessionTimersRefreshMethod = 'UPDATE';
+  DartSIP_C.SipMethod get sessionTimersRefreshMethodEnum {
+    switch (sessionTimersRefreshMethod.toUpperCase()) {
+      case 'INVITE':
+        return DartSIP_C.SipMethod.INVITE;
+      case 'UPDATE':
+      default:
+        return DartSIP_C.SipMethod.UPDATE;
+    }
+  }
 }