[spec] Limit valid encodings of text/gemini to UTF-8

[Sean Conner]

It was thus said that the Great Solderpunk once stated:
> > Le lundi 28 décembre 2020, 14:16:27 CET Philip Linde a écrit :
> 
> > > I understand the need for other document types to take other character
> > > encodings. For example, I have a collection of old text files in IBM437
> > > encoding. For text/gemini, we pretty much have a blank slate, though,
> > > and I see no reason that it should extend to support arbitrary
> > > encodings when limiting to UTF-8 creates a much simpler situation for
> > > implementers and is already the unspoken standard.
> 
> The spec says that "Compliant clients MUST support UTF-8-encoded text/*
> responses.  Clients MAY optionally support other encodings".  

  I would ammend that to read "Compliant clients MUST support UTF-8 and
US-ASCII encoded text/* reponses." This is because US-ASCII is a proper
subset of UTF-8, and any valid ASCII file is also a valid UTF-8 file.  I'm
thinking here of automated MIME detection (ala libmagic) that might return a
MIME type of 'text/plain; charset=us-ascii' for a text file.

  -spc