forward all refer to feature server (#156)

* forward all refer to feature server

* forwawrd extra custom header

lib/call-session.js

@@ -780,12 +780,19 @@ class CallSession extends Emitter {
   async _onRefer(dlg, req, res) {
     /* REFER coming in from a sip device, forward to feature server */
     try {
+      const customHeaders = Object.keys(req.headers)
+        .filter((h) => h.toLowerCase().startsWith('x-'))
+        .reduce((acc, h) => {
+          acc[h] = req.get(h);
+          return acc;
+        }, {});
       const response = await dlg.other.request({
         method: 'REFER',
         headers: {
           'Refer-To': req.get('Refer-To'),
           'Referred-By': req.get('Referred-By'),
-          'User-Agent': req.get('User-Agent')
+          'User-Agent': req.get('User-Agent'),
+          ...customHeaders
         }
       });
       res.send(response.status, response.reason);