goproxy buffers SSE (content-type: text/event-stream)

[ChristianSchenk711]

When using goproxy with SSE, the events are buffered, rendering that useless.

Scenario:

• set up an event source for server side events, e.g. like in https://jasonbutz.info/2018/08/server-sent-events-with-node/
• curl it through goproxy (or use a proper SSE client in javascript using an EventSource)

Problem: the goproxy will always return 80 messages as a bunch and buffer inbetween. That renders SSE unusable through goproxy, because the point of SSE is to informed about each individual message.

I would suggest to handle SSE with an explicit Flush, e.g. like that in proxy.go (line 150):

	var copyWriter io.Writer = w
	if w.Header().Get("content-type") == "text/event-stream" {
		copyWriter = NewFlushWriter(w)
	}
	nr, err := io.Copy(copyWriter , resp.Body)

The FlushWriter would just overwrite "write" and call a flush afterwards:

type FlushWriter struct {
	w      io.Writer
}

func NewFlushWriter(w io.Writer) *FlushWriter {
	return &FlushWriter{w: w}
}

func (w *FlushWriter) Write(b []byte) (int, error) {
	bytes, err := w.w.Write(b)
	if f, ok := w.w.(http.Flusher); ok {
		f.Flush()
	} 
	return bytes, err
}

I had initially reported that fix at eclipse-che/che#15942 but then realized that this is actually in goproxy.