Knock mode does not remove Proxy-Agent header

[wzxjohn]

Proxy-Agent Header is added at:

x/handler/http/handler.go

Lines 257 to 259 in 379d9a7

	if resp.Header.Get("Proxy-Agent") == "" {
	resp.Header.Set("Proxy-Agent", h.md.proxyAgent)
	}

When use Knock mode to enable Probing Resistance, the Proxy-Agent header is still exist, which indicates that the server is a proxy server.

Maybe this header should be added after proxy authenticate to avoid this leak.