Add tests for running on non standard ports

If I'm not missing something original issue is a bit outdated already. It mentions 3 lines of code working with r.Host and assumes that they causes a problem with host. As I understand original intention was to replace these r.Host with getHost func.

I found these lines:

• https://gitlab.com/gitlab-org/gitlab-pages/blob/415f4224b04c631588fba386687e69b47cae9be4/internal/domain/domain.go#L296
• https://gitlab.com/gitlab-org/gitlab-pages/blob/415f4224b04c631588fba386687e69b47cae9be4/internal/domain/domain.go#L306
• https://gitlab.com/gitlab-org/gitlab-pages/blob/master/internal/domain/domain.go#L378

First two already have been fixed, and the third one is actually correct.

So I just added some tests to insure that domain package work with non standard ports correctly.

Closes #164 (closed)