title: #validate does not seem to work correctly with :on => :create/:update
body: <p>Greetings,</p>
  
  <p>I have a custom validation in my exemplary Movie model:</p>
  
  <pre><code>class Movie &lt; ActiveRecord::Base
    validate :it, :on =&gt; :create
  
    private
  
    def it
      self.errors.add 'foo', 'bar'
    end
  end
  </code></pre>
  
  <p>This works on movie creation but <em>also</em> on updating an existing movie. <code>:on =&gt; :update</code> will also work for both. Might that be a bug or am I missing something?</p>
  
  <p>Best regards</p>
  
  <p>Tobias</p>