title: In a migration, how do you change the column type to text without specifying the :limit?
body: <p>I want to change a column type from <code>:string</code> to <code>:text</code></p>
  
  <p>The problem is that if I simply do:</p>
  
  <pre><code>change_column :questions, :content, :text
  </code></pre>
  
  <p>Then the schema is rewritten to:</p>
  
  <pre><code>t.string   "content",     :limit =&gt; 255
  </code></pre>
  
  <p>i.e. it takes the length that was implicit when it was a <code>string</code></p>
  
  <p>How can I do the migration and specify that the <code>:limit</code> should not be set such that the schema reads:</p>
  
  <pre><code>t.string   "content"
  </code></pre>
  
  <p>I have tried setting an arbitrary limit of 10,000 but a) that feels inefficient and b) it makes the down migration attempt to set the <code>:text</code> limit to 10,000.</p>
  
  <p>What option can I pass to the <code>change_column</code> method to ensure it simply uses the default limit?</p>