Cache-Control is missing from the spec

[toomim]

Our current implementations (both braidify and braid-protocol) both set the cache-control: no-cache, no-transform header, but it looks like this is no longer in the spec.

It looks like it used to exist, as I see lots of discussion in issues mentioning it: https://github.com/braid-org/braid-spec/issues/16, https://github.com/braid-org/braid-spec/issues/69, https://github.com/braid-org/braid-spec/issues/74, https://github.com/braid-org/braid-spec/issues/17.

I think we should add it back into the spec, but I suppose first we might want to discover where in the spec's git history it disappeared.