title: default for serialized column in activerecord migration
body: <p>so i have a serialized column :dimensions, and in my migration, i would like to set the field to be a default hash.</p>
  
  <p>i have tried...</p>
  
  <pre><code>create_table :shipping_profiles do |t|
        t.string      :dimensions_in, :default =&gt; {:width =&gt; 0, :height =&gt; 0, :depth =&gt; 0}
  </code></pre>
  
  <p>and just</p>
  
  <pre><code>t.string :dimensions_in, :default =&gt; Hash.new()
  </code></pre>
  
  <p>but the fields end up null.  how can i set a default serialized object for this field on creation, or at least make sure my serialize attribute is always a hash?</p>