proxy: unsupported upgrade fallback ignores backend headers

[AlexanderYastrebov]

Whenever Skipper sees connection upgrade headers it assumes connection will be upgraded and when status is not 101 it tries to serve the backend response to the client.
In doing so it completely ignores backend response headers

skipper/proxy/upgrade.go

Lines 125 to 134 in c3b1657

	if resp.StatusCode != http.StatusSwitchingProtocols {
	log.Debugf("Got invalid status code from backend: %d", resp.StatusCode)
	w.WriteHeader(resp.StatusCode)
	_, err := io.Copy(w, resp.Body)
	if err != nil {
	log.Errorf("Error writing body to client: %s", err)
	return
	}
	return
	}

which results in Content-Type: text/plain; charset=utf-8, missing backend headers and Skipper branding

skipper/proxy/proxy.go

Lines 929 to 934 in c3b1657

	// addBranding overwrites any existing `X-Powered-By` or `Server` header from headerMap
	func addBranding(headerMap http.Header) {
	if headerMap.Get("Server") == "" {
	headerMap.Set("Server", "Skipper")
	}
	}

Tentatively this code path should reuse proxy code to serve a backend response.

See related HTTP/1.1 TLS Upgrade (RFC-2817) should not be default.

[AlexanderYastrebov]

Handling of non 101 status was added by #921 and changed to send the body by #1291

[AlexanderYastrebov]

As a workaround for https://issues.apache.org/jira/browse/HTTPCLIENT-2344 one may add dropRequestHeader("Connection") -> dropRequestHeader("Upgrade") filters.

It would be handy to drop a specific Connection: Upgrade and e.g. Upgrade: TLS/1.0 values only so I've created #3457

[szuecs]

Interesting issue!