serialize custom emoji reactions properly for AP

Akkoma and possibly others expect the `tag` field
in an EmojiReact activity to be an array, not just
a single object, so it's being wrapped into one
now.  I'm not entirely sure whether this is the
idiomatic way of doing it tbh, but it works fine.

app/serializers/activitypub/emoji_reaction_serializer.rb

@@ -3,8 +3,7 @@
 class ActivityPub::EmojiReactionSerializer < ActivityPub::Serializer
   attributes :id, :type, :actor, :content
   attribute :virtual_object, key: :object
-
-  has_one :custom_emoji, key: :tag, serializer: ActivityPub::EmojiSerializer, unless: -> { object.custom_emoji.nil? }
+  attribute :custom_emoji, key: :tag, unless: -> { object.custom_emoji.nil? }
 
   def id
     [ActivityPub::TagManager.instance.uri_for(object.account), '#emoji_reactions/', object.id].join
@@ -31,4 +30,10 @@ class ActivityPub::EmojiReactionSerializer < ActivityPub::Serializer
   end
 
   alias reaction content
+
+  # Akkoma (and possibly others) expect `tag` to be an array, so we can't just
+  # use the has_one shorthand because we need to wrap it into an array manually
+  def custom_emoji
+    [ActivityPub::EmojiSerializer.new(object.custom_emoji).serializable_hash]
+  end
 end