FIX: inviting a user again after the first invite expires will create a new invite

app/serializers/invite_serializer.rb

@@ -1,10 +1,14 @@
 class InviteSerializer < ApplicationSerializer
 
-  attributes :email, :created_at, :redeemed_at
+  attributes :email, :created_at, :redeemed_at, :expired
   has_one :user, embed: :objects, serializer: InvitedUserSerializer
 
   def include_email?
     !object.redeemed?
   end
 
+  def expired
+    object.expired?
+  end
+
 end