Specification clearly states:
URL may be absolute or relative. If relative, it should be resolved
against the URL used in the original request.
So "gemini://foo.bar/baz" with "doo" relative URL must give
"gemini://foo.bar/baz/doo" URL. But initial Gemini's Python demo
(https://tildegit.org/solderpunk/gemini-demo-1.git) performs urljoin
call, that will give "gemini://foo.bar/doo". Seems that many gemsites
expect that behaviour from the client.