Add req.host to targetURL when redirecting to https (#668)

* Add req.host to targetURL when redirecting to https

The req.URL.Host might not be present when redirecting to https if the
given req.URL is something like "/". In such scenario, the req.Host is
still present and valid.

This commit adds the original req.Host to the targetURL before returning
the 308 status, to avoid having a `Location: https:///` in the response.

* Bring back empty line

* Wrap the setting of targetURL.Host in a condition

* Add a comment to the test explaining why the redirectURL includes example.com

* Add changelog entry