Problem running app generator standalone

I'm trying to use the AppGenerator inside an engine, and the copy_file method is going directly to fileutils instead of using the thor actions method. The error is on master, it works on 3.0.5.

rake generate looks something like this:

require 'rails/generators'
require 'rails/generators/rails/app/app_generator'

class TestAppGenerator < Rails::Generators::AppBase
  source_root File.expand_path("../templates", __FILE__)

  def run_rails_new
    invoke Rails::Generators::AppGenerator, ["spec/dummy"]
  end
  ...
end
TestAppGenerator.start

% rake generate --trace
(in /Users/steve/code/engine_test)
** Invoke generate (first_time)
** Execute generate
       exist  
rake aborted!
wrong number of arguments (1 for 2)
/Users/steve/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/1.9.1/fileutils.rb:471:in `copy_file'
/Users/steve/code/rails/railties/lib/rails/generators/rails/app/app_generator.rb:33:in `readme'
/Users/steve/code/rails/railties/lib/rails/generators/app_base.rb:85:in `build'
/Users/steve/code/rails/railties/lib/rails/generators/rails/app/app_generator.rb:181:in `create_root_files'
/Users/steve/.rvm/gems/ruby-1.9.2-p180/gems/thor-0.14.6/lib/thor/task.rb:22:in `run'

Using git diff I've tracked it down to the removal of a bunch of methods being defined in 3.0.5 which are removed in master.

git diff v3.0.5..master railties/lib/rails/generators/rails/app/app_generator.rb

0904e8256864239f673bf91fce1cfffb9345ee61 - This is the commit that stops it working, although I find it strange that it's from October last year and it's not in the 3.0.5 release. Maybe I'm looking at the wrong commit.

In any case adding the method definitions back solves the problem I'm having. I can't figure out why it fails in this case but works fine through the rails command though...

Thanks