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

[hmoragrega]

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

[hmoragrega]

I see there's a fix in master already but not released yet