Pass validated record to the custom message proc

I've added a small improvement to the validation errors processing.
It allows to pass the validated record to the proc for generating a custom error message. For instance we could use it this way:

class Subscription < ActiveRecord::Base

  validates :city, :presence => true
  validates_uniqueness_of :email, :scope => [:city_id], 
    :message => proc { |s| "You're already subscribed to #{city.name}" }

end