Specify content-type for referrers response

Signed-off-by: Brandon Mitchell <git@bmitch.net>

conformance/03_discovery_test.go

@@ -291,6 +291,7 @@ var test03ContentDiscovery = func() {
 				resp, err := client.Do(req)
 				Expect(err).To(BeNil())
 				Expect(resp.StatusCode()).To(Equal(http.StatusOK))
+				Expect(resp.Header().Get("Content-Type")).To(Equal("application/vnd.oci.image.index.v1+json"))
 
 				var index index
 				err = json.Unmarshal(resp.Body(), &index)
@@ -305,6 +306,7 @@ var test03ContentDiscovery = func() {
 				resp, err := client.Do(req)
 				Expect(err).To(BeNil())
 				Expect(resp.StatusCode()).To(Equal(http.StatusOK))
+				Expect(resp.Header().Get("Content-Type")).To(Equal("application/vnd.oci.image.index.v1+json"))
 				if h := resp.Header().Get("Docker-Content-Digest"); h != "" {
 					Expect(h).To(Equal(configs[4].Digest))
 				}
@@ -324,6 +326,7 @@ var test03ContentDiscovery = func() {
 				resp, err := client.Do(req)
 				Expect(err).To(BeNil())
 				Expect(resp.StatusCode()).To(Equal(http.StatusOK))
+				Expect(resp.Header().Get("Content-Type")).To(Equal("application/vnd.oci.image.index.v1+json"))
 				if h := resp.Header().Get("Docker-Content-Digest"); h != "" {
 					Expect(h).To(Equal(configs[4].Digest))
 				}

spec.md

@@ -555,6 +555,7 @@ If the registry supports the referrers API, the registry MUST NOT return a `404
 If the request is invalid, such as a `<digest>` with an invalid syntax, a `400 Bad Request` MUST be returned.
 
 Upon success, the response MUST be a JSON body with an image index containing a list of descriptors.
+The `Content-Type` header MUST be set to `application/vnd.oci.image.index.v1+json`.
 Each descriptor is of an image manifest in the same `<name>` namespace with a `subject` field that specifies the value of `<digest>`.
 The descriptors MUST include an `artifactType` field that is set to the value of the `artifactType` in the image manifest, if present.
 If the `artifactType` is empty or missing in the image manifest, the value of `artifactType` MUST be set to the config descriptor `mediaType` value.