title: Rails serializable_hash error for nil columns
body: <p>Having a bug where I cannot render json (I am getting thrown around a TypeError: nil is not a symbol) if one of my columns is nil.</p>
  
  <p>Something like User.first.serializable_hash throws an error like this:</p>
  
  <pre><code>TypeError: nil is not a symbol
      from /usr/local/rvm/gems/ruby-2.0.0-p247@global/gems/activemodel-4.0.3/lib/active_model/serialization.rb:108:in `block in serializable_hash'
      from /usr/local/rvm/gems/ruby-2.0.0-p247@global/gems/activemodel-4.0.3/lib/active_model/serialization.rb:108:in `each'
      from /usr/local/rvm/gems/ruby-2.0.0-p247@global/gems/activemodel-4.0.3/lib/active_model/serialization.rb:108:in `serializable_hash'
      from /usr/local/rvm/gems/ruby-2.0.0-p247@global/gems/activerecord-4.0.3/lib/active_record/serialization.rb:17:in `serializable_hash'
  </code></pre>
  
  <p>Anyone ever run into this problem? I know I can use activemodel serializers but I would have to put one on for every single model (and i can't turn it off for a specific controller).</p>
  
  <p>Also: I am using rails 4 to build an api and using mysql2 adapter gem if that matters</p>