x-forwarded-host overwrite for mutli level proxies (http-party#1267)

With more than 1 proxy the original host was lost, now it will be passed down the proxy chain

lib/http-proxy/passes/web-incoming.js

@@ -82,7 +82,7 @@ module.exports = {
         values[header];
     });
 
-    req.headers['x-forwarded-host'] = req.headers['host'] || '';
+    req.headers['x-forwarded-host'] = req.headers['x-forwarded-host'] || req.headers['host'] || '';
   },
 
   /**