Requiring schemes in Gemini requests

[Adnan Maolood]

Not requiring schemes in Gemini requests has led to lots of confusion.
I propose that Gemini requests must contain a scheme.

This will make things simpler for server authors, who will not have to
check for the scheme "gemini" as well as check for an empty scheme.
Some servers do not properly handle this anyways.

Linking to a URL without a scheme should still work. It would be up to
the client to resolve the URL reference before making the request. This
functionality should already be provided by whatever library the client
uses to handle relative URLs.