XRID proposal for YADIS

[Martin Atkins]

Does XRID have a registered MIME type?

While the current spec doesn't directly depend on this, my proposed
changes do. The capabilities document must have a distinctive type which
can be sniffed for in the Accept header.

Even without my proposed changes, text/xml isn't a very reliable MIME
type to use to see if the response is a capabilities document and so the
consumer would have to just try parsing it rather than shortcutting to
failure.