title: Rails attr_readonly doesn't work
body: <p>According to <a href="https://stackoverflow.com/questions/7376659/make-attributes-mass-assignable-only-during-creation/7376686">this question</a> and the <a href="http://apidock.com/rails/ActiveRecord/ReadonlyAttributes/ClassMethods/attr_readonly" rel="nofollow noreferrer">documentation</a> of <code>attr_readonly</code> the following should be possible:</p>
  
  <pre><code>class MyModel &lt; ActiveRecord::Base
    attr_accessible :foo
    attr_readonly :bar
  end
  
  m = MyModel.create(foo: '123', bar: 'bar') # Should work
  m.update_attributes(bar: 'baz')            # Should not work
  </code></pre>
  
  <p>However the first one fails, saying that I can't mass-assign <code>bar</code>. What am I mising?</p>