Feature/incoming refer (#25)

* handle incoming REFER and send on to the FS

* clarity

lib/call-session.js

@@ -618,8 +618,20 @@ Duration=${payload.duration} `
       }
     }
     else {
-      // TODO: forward on to feature server
-      res.send(501);
+      /* REFER coming in from a sip device, forward to feature server */
+      try {
+        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')
+          }
+        });
+        res.send(response.status, response.reason);
+      } catch (err) {
+        this.logger.error({err}, 'CallSession:_onRefer: error handling incoming REFER');
+      }
     }
   }