attr_readonly not set when counter_cache set to true

Due to the class loading feature in Rails, a model class with counter_cache could be loaded before its associated class, which may cause attr_readonly for "[model]_count" field not set for its associated class.

The chances of this occuring in development environment is much higher as codes are reloaded for every request.

A simple test case to trigger this issue:

class Topic
end

class Post
  belongs_to :topic, :counter_cache => true  
end

class CounterCacheAttrReadonlyTest < Test::Unit::TestCase
  def test_attr_readonly_not_set
    Post.create :topic_id => 1
    
    # Failed, attr_readonly for posts_count in Topic not set 
    # because Topic has not been loaded and defined? returns false      
    assert_equal Set.new('posts_count'), Topic.attr_readonly 
  end
end

Attached is the patch. Thanks.