fix #2368, which causes system lacked sufficient buffer space (#2372)

* fix #2368

* add CloseIdleConnections at WriteToFileMultiThreading

internal/download/download.go

@@ -137,6 +137,7 @@ func (r Request) Bytes() ([]byte, error) {
 		return nil, err
 	}
 	defer rd.Close()
+	defer r.client().CloseIdleConnections()
 	return io.ReadAll(rd)
 }
 
@@ -147,6 +148,7 @@ func (r Request) JSON() (gjson.Result, error) {
 		return gjson.Result{}, err
 	}
 	defer rd.Close()
+	defer r.client().CloseIdleConnections()
 
 	var sb strings.Builder
 	_, err = io.Copy(&sb, rd)
@@ -174,6 +176,7 @@ func (r Request) WriteToFile(path string) error {
 		return err
 	}
 	defer rd.Close()
+	defer r.client().CloseIdleConnections()
 	return writeToFile(rd, path)
 }
 
@@ -183,6 +186,7 @@ func (r Request) WriteToFileMultiThreading(path string, thread int) error {
 		return r.WriteToFile(path)
 	}
 
+	defer r.client().CloseIdleConnections()
 	limit := r.Limit
 	type BlockMetaData struct {
 		BeginOffset    int64