Http2ClientConnection overwrites existing X_FORWARDED_FOR header

In the Http2ClientConnection.sendRequest method if the peer is not null, the existing X_FORWARDED_FOR header is overwitten. This leads to information loss in an architecture where for example the we have an undertow load balancer with undertow nodes using HTTP2.

String peer = request.getAttachment(ProxiedRequestAttachments.REMOTE_HOST);
if(peer != null) {
    request.getRequestHeaders().put(Headers.X_FORWARDED_FOR, peer);
}