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>
This changeset provides a common http handler for serving errcodes. This should
unify http responses across webservices in the face of errors.
Several type assertions have been added, as well, to ensure the error interface
is implemented.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
