Fix content type for schema1 signed manifests

The Payload function for schema1 currently returns a signed manifest,
but indicates the content type is that of a manifest that isn't signed.

Note that this breaks compatibility with Registry 2.3 alpha 1 and
Docker 1.10-rc1, because they use the incorrect content type.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

notifications/http_test.go

@@ -75,12 +75,12 @@ func TestHTTPSink(t *testing.T) {
 		{
 			statusCode: http.StatusOK,
 			events: []Event{
-				createTestEvent("push", "library/test", schema1.MediaTypeManifest)},
+				createTestEvent("push", "library/test", schema1.MediaTypeSignedManifest)},
 		},
 		{
 			statusCode: http.StatusOK,
 			events: []Event{
-				createTestEvent("push", "library/test", schema1.MediaTypeManifest),
+				createTestEvent("push", "library/test", schema1.MediaTypeSignedManifest),
 				createTestEvent("push", "library/test", layerMediaType),
 				createTestEvent("push", "library/test", layerMediaType),
 			},