Fix filter ID not being a string in REST API (#9303)

app/serializers/rest/filter_serializer.rb

@@ -3,4 +3,8 @@
 class REST::FilterSerializer < ActiveModel::Serializer
   attributes :id, :phrase, :context, :whole_word, :expires_at,
              :irreversible
+
+  def id
+    object.id.to_s
+  end
 end