Added two methods: - *Client.CustomHTTPHeaders() map[string]string - *Client.SetCustomHTTPHeaders(headers map[string]string) Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp> Upstream-commit: a754d89b4058d8508cf3cee29f6107b2f93f3001 Component: engine