HTTP2 error on req.headers

I think the req.headers object is slightly different in the Node 8 http2 module.

```~/cdn/node_modules/@dadi/logger/dadi/index.js:186
      if (req.headers.hasOwnProperty('x-forwarded-for')) {
                      ^

TypeError: req.headers.hasOwnProperty is not a function
```

Could this be `if(req.headers['x-forwarded-for'])` instead?