move AdminDetailedUserSerializer additions out of plugin.rb

config/initializers/patch_admin_detailed_user_serializer.rb

@@ -0,0 +1,2 @@
+AdminDetailedUserSerializer.include MozillaIAM::AdminDetailedUserSerializerExtensions
+AdminDetailedUserSerializer.attributes :mozilla_iam

lib/mozilla_iam.rb

@@ -1,5 +1,6 @@
 require_relative 'mozilla_iam/engine'
 
+require_relative 'mozilla_iam/admin_detailed_user_serializer_extensions'
 require_relative 'mozilla_iam/api'
 require_relative 'mozilla_iam/person_api'
 require_relative 'mozilla_iam/management_api'

lib/mozilla_iam/admin_detailed_user_serializer_extensions.rb

@@ -0,0 +1,13 @@
+module MozillaIAM
+  module AdminDetailedUserSerializerExtensions
+
+    def mozilla_iam
+      object.custom_fields.select do |k, v|
+        k.start_with?('mozilla_iam')
+      end.map do |k, v|
+        [k.sub('mozilla_iam_', ''), v]
+      end.to_h
+    end
+
+  end
+end

plugin.rb

@@ -22,15 +22,3 @@
               message: 'Log In / Sign Up',
               authenticator: MozillaIAM::Authenticator.new('auth0', trusted: true),
               full_screen_login: true)
-
-after_initialize do
-
-  add_to_serializer(:AdminDetailedUser, :mozilla_iam, false) do
-    object.custom_fields.select do |k, v|
-      k.start_with?('mozilla_iam')
-    end.map do |k, v|
-      [k.sub('mozilla_iam_', ''), v]
-    end.to_h
-  end
-
-end