title: Model.create() throws errors with readonly attributes
body: <p>I have the following model in rails (simplified):</p>
  
  <pre><code>class Phone &lt; ActiveRecord::Base
    include ActiveModel::Validations
    belongs_to :user
    belongs_to :brand
    attr_readonly :user, :brand
    attr_accessible :model, :phone_number
  
    validates :user, :presence =&gt; true
    validates :brand, :presence =&gt; true
    validates :model, :presence =&gt; true
    validates :phone_number, :presence =&gt; true
  end
  </code></pre>
  
  <p>According to the documentation, attr_readonly should allow attributes to be set at creation, but not at update.</p>
  
  <p>However, when I do this:</p>
  
  <p><code>Phone.create(:user =&gt; &lt;existing_user&gt;, :brand =&gt; &lt;existing_brand&gt;, :model =&gt; "Galaxy", :phone_number =&gt; "555-433-5678")</code></p>
  
  <p>I get this error:</p>
  
  <p><code>Can't mass-assign protected attributes user, brand</code></p>
  
  <p>What am I missing?</p>