[mod_proxy] set X-Forwarded-Host (fixes #418)

lighttpd appends the request "Host" to the non-standard header "X-Host"
when acting as a reverse proxy.  This patch additionally appends "Host"
to the non-standard header "X-Forwarded-Host" to match the convention in
some other web servers

x-ref:
  "mod_proxy should send the X-Forwarded-Host header to the proxied server"
  https://redmine.lighttpd.net/issues/418

src/mod_proxy.c

@@ -665,6 +665,7 @@ static int proxy_create_env(server *srv, handler_ctx *hctx) {
 	 * which is NULL if it is not set */
 	if (!buffer_string_is_empty(con->request.http_host)) {
 		proxy_set_header(con, "X-Host", con->request.http_host->ptr);
+		proxy_set_header(con, "X-Forwarded-Host", con->request.http_host->ptr);
 	}
 	proxy_set_header(con, "X-Forwarded-Proto", con->uri.scheme->ptr);