X-Forwarded-For generated header is incorrect if previous values exist

If the request that we have to proxy already has an IP in the `X-Forwarded-For` header the concatenation is wrong

Example instead of
```
1.1.1.1, 8.8.8.8
```
It forwards
```
1.1.1.18.8.8.8
```

Example:

https://play.golang.org/p/lVQNcaZ_kmy


