ocaml-http: enable MDX on lib/http_date.mli
Run mdx on lib/http_date.mli so the {[ ... ]} odoc block now
type-checks. The block had ocamlformat-collapsed [parse ... (* RFC
1123 *) parse ...] runs that the toploop reads as a single chained
function application. Replaced with three explicit
`assert (Option.is_some ...)` lines, one per documented date format.