title: Reversible migration for change_column_default from not having any default in Rails
body: <p>The <a href="http://edgeguides.rubyonrails.org/active_record_migrations.html#changing-columns" rel="noreferrer">Rails guides to active record migrations</a> says that you can do</p>
  
  <pre><code>change_column_default :products, :approved, from: true, to: false
  </code></pre>
  
  <p>I've got a <code>change</code> method in Rails that's similar to the following:</p>
  
  <pre><code>change_column_default :people, :height, from: nil, to: 0
  </code></pre>
  
  <p>with the intention of going from not having any defaults, to having a default of zero.</p>
  
  <p>However, when I try rolling it back, I get</p>
  
  <pre><code>ActiveRecord::IrreversibleMigration: ActiveRecord::IrreversibleMigration
  </code></pre>
  
  <p>Considering I give Rails a <code>from</code> and <code>to</code>, why isn't it accepting it?</p>
  
  <p>I'm using Rails 4.2.0.</p>