This fixes registry endpoints to return the proper `application/json`
content-type for JSON content, also updating spec examples for that.
As per IETF specification and IANA registry [0], the `application/json`
type is a binary media, so the content-type label does not need any
text-charset selector. Additionally, the media type definition
explicitly states that it has no required nor optional parameters,
which makes the current registry headers non-compliant.
[0]: https://www.iana.org/assignments/media-types/application/json
Signed-off-by: Luca Bruno <lucab@debian.org>
15b0204758