cmd/present: fix mixed content warnings when served by HTTPS
cmd/present: fix mixed content warnings when served by HTTPS

Fixes golang/go#11392

Change-Id: Ic7ecf19d88c5a7f1d0ef9bd21476074b242854f0
Reviewed-on: https://go-review.googlesource.com/11534
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cmars/tools,josharian/tools,glasser/golang-tools,runcom/tools,nicot/tools,jdhenke/tools,sshetty10/tools,runcom/tools,cw2018/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,josharian/tools,syreclabs/go-tools,runcom/tools,cmars/tools,josharian/tools,nicot/tools,cw2018/tools,sshetty10/tools,springning/tools,cw2018/tools,golang/tools,argandas/tools,magiccao/tools,golang/tools,springning/tools,argandas/tools,syreclabs/go-tools,matryer/tools,matryer/tools,nicot/tools,sshetty10/tools,golang/tools,glasser/golang-tools,springning/tools,nicot/tools,sshetty10/tools,golang/tools,golang/tools,jdhenke/tools,magiccao/tools,syreclabs/go-tools,josharian/tools,springning/tools,glasser/golang-tools,cmars/tools,jdhenke/tools,jdhenke/tools,cw2018/tools,magiccao/tools,cmars/tools,runcom/tools,matryer/tools,matryer/tools,magiccao/tools,argandas/tools,glasser/golang-tools,argandas/tools