go: html/template: template.HTML strings escaped (Go 1.9 regression)

This has been reported by several users in the wild after we released Hugo 0.27 built on Go 1.9.

I have tried to make a simple standalone and failing test case, not yet successful.

I have, however, a failing test inside Hugo: https://travis-ci.org/gohugoio/hugo/builds/274496313?utm_source=github_status&utm_medium=notification

It passes fine on Go 1.8.3, fails on Go 1.9 and tip.

See https://github.com/gohugoio/hugo/pull/3878

The gist of it seems to be:

• HTML strings put into the template context, i.e. data fetched from external resources such as Twitter (Tweets) and Instagram.
• These are wrapped as template.HTML types to mark them as safe.

Note that we have had plenty of similar and passing tests in Hugo, so there is a corner case here that I don’t fully understand.

I will go back to using Go 1.8.3 (where I can).