Ensure that after_generate_callbacks are looked up through accessor

This ends up being relevant when `Rails::Generators.invoke` is called, but `Rails::Generators.configure!` is not, which can happen in unit tests.

Author: pvande

railties/lib/rails/generators.rb

@@ -319,7 +319,7 @@ def file_lookup_paths # :doc:
 
         def run_after_generate_callback
           if defined?(@@generated_files) && !@@generated_files.empty?
-            @after_generate_callbacks.each do |callback|
+            after_generate_callbacks.each do |callback|
               callback.call(@@generated_files)
             end
             @@generated_files = []