Generate valid sql for ActiveRecord::Reflection with polymorphic aspect through dependent_conditions method

When I do the following an error is raised:

class Post < ActiveRecord::Base
  has_many :taggings, :as => :taggable, :dependent => :delete_all
end

Post.create!
Post.destroy

I get something like this:

Mysql::Error: Unknown column 'taggable' in 'where clause': DELETE FROM `taggings` WHERE ((taggable = 30) AND (taggable_type = 'Post')) (ActiveRecord::StatementInvalid)

The apparent problem is a defect in the dependent_conditions method of the Reflection class, which generates invalid sql for polymorphic associations. I have attached a patch with a test. This was not an issue in the 2.3 branch. Hopefully it can be resolved quickly?