gddo-server: change redirects to 301 Change-Id: Ie2c31f719a29d3cd2eb7b449bf9908fae875b168 Reviewed-on: https://go-review.googlesource.com/c/gddo/+/289730 Reviewed-by: Jamal Carvalho <jamal@golang.org> Trust: Julie Qiu <julie@golang.org>