Adding configuration option session_timers_force_refresher (versatica#671)

In environment where the WebRTC client is connected via a client-side
only initiated connection, it makes sense to force JsSIP to be
the refresher of session timer. All it takes is basically to add
the "refresher=uac" parameter to the Session-Expires HF in the initial INVITE

Author: kkozlik

lib/Config.js

@@ -25,9 +25,10 @@ exports.settings = {
   use_preloaded_route : false,
 
   // Session parameters.
-  session_timers                : true,
-  session_timers_refresh_method : JsSIP_C.UPDATE,
-  no_answer_timeout             : 60,
+  session_timers                 : true,
+  session_timers_refresh_method  : JsSIP_C.UPDATE,
+  session_timers_force_refresher : false,
+  no_answer_timeout              : 60,
 
   // Registration parameters.
   register         : true,
@@ -233,6 +234,14 @@ const checks = {
       }
     },
 
+    session_timers_force_refresher(session_timers_force_refresher)
+    {
+      if (typeof session_timers_force_refresher === 'boolean')
+      {
+        return session_timers_force_refresher;
+      }
+    },
+
     password(password)
     {
       return String(password);

lib/RTCSession.js

@@ -358,7 +358,7 @@ module.exports = class RTCSession extends EventEmitter
     extraHeaders.push('Content-Type: application/sdp');
     if (this._sessionTimers.enabled)
     {
-      extraHeaders.push(`Session-Expires: ${this._sessionTimers.defaultExpires}`);
+      extraHeaders.push(`Session-Expires: ${this._sessionTimers.defaultExpires}${this._ua.configuration.session_timers_force_refresher ? ';refresher=uac' : ''}`);
     }
 
     this._request = new SIPMessage.InitialOutgoingInviteRequest(