add method to check if association is eager loaded

To optimize sql queries for an application, it is helpful to see if associations that will be used are eager loaded. A simple method will allow users to easily check this. We implemented a version that would be helpful for other users.

class ActiveRecord::Base
  # Returns true if association has been eager loaded; false otherwise.
  # ex. model.eager_loaded?(:association)
  def eager_loaded?(association)
    !!self.instance_variable_get("@#{association}")
  end
end