Don't close idle connections when context was cancelled

If the user no longer wants to wait, it's not a reason to drop all keep-alive connections.

client.go

@@ -653,7 +653,6 @@ func (c *Client) Do(req *Request) (*http.Response, error) {
 		select {
 		case <-req.Context().Done():
 			timer.Stop()
-			c.HTTPClient.CloseIdleConnections()
 			return nil, req.Context().Err()
 		case <-timer.C:
 		}