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/models/media_metadata.rb

@@ -32,4 +32,8 @@ class MediaMetadata < ApplicationRecord
   def frame_delays
     metadata["Ugoira:FrameDelays"].to_a
   end
+
+  def self.available_includes
+    [:media_asset]
+  end
 end