check HTTP headers presence

`HTTP_REFERER` and `HTTP_USER_AGENT` are optional headers from client.
This fix such notices like:
```
Notice: Undefined index: HTTP_REFERER in /var/www/test/vendor/codeception/phpbuiltinserver/src/Codeception/Extension/Router.php on line 28
Notice: Undefined index: HTTP_USER_AGENT in /var/www/test/vendor/codeception/phpbuiltinserver/src/Codeception/Extension/Router.php on line 29
```