media metadata: fix exception in /media_metadata.json?only=media_asset.

Fix NameError exception caused by the MediaMetadataController trying to lookup "MediaMetadatum" as the model name.

app/controllers/media_metadata_controller.rb

@@ -5,6 +5,6 @@ class MediaMetadataController < ApplicationController
 
   def index
     @media_metadata = authorize MediaMetadata.visible(CurrentUser.user).paginated_search(params, count_pages: true)
-    respond_with(@media_metadata)
+    respond_with(@media_metadata, model: "MediaMetadata")
   end
 end