filer, s3: add http status 206 as late as possible

fix https://github.com/chrislusf/seaweedfs/issues/1746

weed/server/volume_server_handlers_read.go

@@ -261,10 +261,13 @@ func writeResponseContent(filename, mimeType string, rs io.ReadSeeker, w http.Re
 		return nil
 	}
 
-	processRangeRequest(r, w, totalSize, mimeType, func(writer io.Writer, offset int64, size int64) error {
+	processRangeRequest(r, w, totalSize, mimeType, func(writer io.Writer, offset int64, size int64, httpStatusCode int) error {
 		if _, e = rs.Seek(offset, 0); e != nil {
 			return e
 		}
+		if httpStatusCode != 0 {
+			w.WriteHeader(httpStatusCode)
+		}
 		_, e = io.CopyN(writer, rs, size)
 		return e
 	})