godoc/proxy: use HTTPS when bouncing through to the playground
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 <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools