Preconnect links are used to establish a server connection without loading a
specific resource yet. Not always do these links point to a URL that should
return a 200, and they are not user-facing, i.e. they don't show up in the
final rendered version of a page.

Therefore, I think we should them at all; not even in `--include-verbatim`
mode, as they might not point to a valid resource.

Fixes #897