UNDERTOW-297 Set the X-Forwarded-For header when proxying

Update connection logic to append to the X-Forwarded-For header if it is present. Otherwise, just add the header.
We can certainly just append the new host as an option. The challenge is that the HttpClientConnection and SpdyClientConnection are a few levels deep from the ProxyHandler. Would it make sense that one can add an option to something like LoadBalancingProxyClient which then the connections will pickup on.

Perhaps we add a new Undertow option, something like PROXY_APPEND_X_FORWARDED. If present and true then the host will be appended to X-Forwarded-For header rather then reset like it is today.