Don't throw if res.req.path isn't defined.

I’m using [node-replay](https://github.com/assaf/node-replay) to mock http requests / responses from mailgun when testing. Replay’s proxied `res` object doesn’t have a `req` property, so this line throws. This PR fixes this case. Generally, I think it couldn't hurt to check if the property exists. The Node docs don’t mention HttpResponse having a `req` so perhaps it is best not to rely on it existing.