Make SplitHTTP work under CF Argo (#3479)

* Make SplitHTTP work under CF Argo

* Update hub.go

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Co-Authored-By: 风扇滑翔翼 <Fangliding.fshxy@outlook.com>

transport/internet/splithttp/hub.go

@@ -161,6 +161,9 @@ func (h *requestHandler) ServeHTTP(writer http.ResponseWriter, request *http.Req
 
 		// magic header instructs nginx + apache to not buffer response body
 		writer.Header().Set("X-Accel-Buffering", "no")
+		// magic header to make the HTTP middle box consider this as SSE to disable buffer
+		writer.Header().Set("Content-Type", "text/event-stream")
+
 		writer.WriteHeader(http.StatusOK)
 		// send a chunk immediately to enable CDN streaming.
 		// many CDN buffer the response headers until the origin starts sending