Check media types when unmarshalling manifests

When unmarshalling manifests from JSON, check that the MediaType field
corresponds to the type that we are unmarshalling as. This makes sure
that when we retrieve a manifest from the manifest store, it will have
the same type as it was handled as before storing it in the manifest
store.

Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>

manifest/schema2/manifest.go

@@ -116,6 +116,12 @@ func (m *DeserializedManifest) UnmarshalJSON(b []byte) error {
 		return err
 	}
 
+	if manifest.MediaType != MediaTypeManifest {
+		return fmt.Errorf("mediaType in manifest should be '%s' not '%s'",
+			MediaTypeManifest, manifest.MediaType)
+
+	}
+
 	m.Manifest = manifest
 
 	return nil