Add text/event-stream for long running http connections (#7909)

When MinIO is behind a proxy, proxies end up killing
clients when no data is seen on the connection, adding
the right content-type ensures that proxies do not come
in the way.

cmd/object-handlers.go

@@ -2280,6 +2280,7 @@ func (api objectAPIHandlers) CompleteMultipartUploadHandler(w http.ResponseWrite
 		w.Write(encodedErrorResponse)
 		w.(http.Flusher).Flush()
 	}
+	w.Header().Set(xhttp.ContentType, "text/event-stream")
 	w = &whiteSpaceWriter{ResponseWriter: w, Flusher: w.(http.Flusher)}
 	completeDoneCh := sendWhiteSpace(ctx, w)
 	objInfo, err := completeMultiPartUpload(ctx, bucket, object, uploadID, completeParts, opts)