Check if properRoute is found before acting on it

In the cases where the request route is not matched to any Koa route,
bail out before trying to format the path string.

Issue: PayU#59

src/koa-middleware.js

@@ -33,7 +33,7 @@ class KoaMiddleware {
         }
     }
 
-    _handleResponse (ctx) {
+    _handleResponse(ctx) {
         const responseLength = parseInt(ctx.response.get('Content-Length')) || 0;
 
         const route = this._getRoute(ctx) || 'N/A';
@@ -93,6 +93,12 @@ class KoaMiddleware {
             }
         });
 
+        // If proper route is not found, send an undefined route
+        // The caller is responsible for setting a default "N/A" route in this case
+        if (!properRoute) {
+            return undefined;
+        }
+
         let route = properRoute.path;
         route = route.endsWith('/') ? route.substring(0, route.length - 1) : route;
         return route;