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.
I have reviewed the logic again and see where I can add a parameter to the `ProxyHandler` constructor that allows for setting up the X-Forwarded-By header correctly. The REMOTE_HOST  attachment would have to be altered to either carry the X-FORWARDED contents and just set them in `HttpClientConnection` or `SpdyClientConnection`. One alternative would be to add an additional attachment to handle this. After looking at the history, I saw that in the past the ProxyHandler managed all of this and just set the request header appropriately and then the client connections did nothing.
If we were to go back to that then perhaps that would be cleaner, but I am sure there were distinct reasons to move that logic out in to `SpyClientConnection` and `HttpClientConnection`.

NOTE: Neither of the scenarios above address the issue when using Ajp, unless we set the request headers in the ProxyHandler and then the header are already copied in `AjpClientRequestClientStreamSinkChannel`.