registry: fix binary JSON content-type

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>

registry/client/repository_test.go

@@ -80,7 +80,7 @@ func addTestFetch(repo string, dgst digest.Digest, content []byte, m *testutil.R
 func addTestCatalog(route string, content []byte, link string, m *testutil.RequestResponseMap) {
 	headers := map[string][]string{
 		"Content-Length": {strconv.Itoa(len(content))},
-		"Content-Type":   {"application/json; charset=utf-8"},
+		"Content-Type":   {"application/json"},
 	}
 	if link != "" {
 		headers["Link"] = append(headers["Link"], link)
@@ -1062,7 +1062,7 @@ func TestObtainsErrorForMissingTag(t *testing.T) {
 			StatusCode: http.StatusNotFound,
 			Body:       errBytes,
 			Headers: http.Header(map[string][]string{
-				"Content-Type": {"application/json; charset=utf-8"},
+				"Content-Type": {"application/json"},
 			}),
 		},
 	})