Fix: X-Forwarded-For double-use and no comma

[stevekerrison]

The forwarder attempts to extend, or create, the X-Forwarded-For header. However, the logic is slightly broken and breaks the header. It does:

1. Initialise xff to the original header value.
2. If it's non-empty, append , to it.
3. Append the original header again, plus clammit's own IP without any comma separation

So, in this pull request I've dealt with both of those issues with a simple modification.

HTH,
Steve.

[vjt]

Great, thank you!