godoc/proxy: use HTTPS when bouncing through to the playground

Fixes golang/go#15692

Change-Id: I0d0731b249de9399b972c8ee9cf9e0f355d4fcc0
Reviewed-on: https://go-review.googlesource.com/23241
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed