title: Change format of attribute in serializer
body: <p>I'm using <a href="https://github.com/rgeo/activerecord-postgis-adapter" rel="nofollow noreferrer">PostGIS ActiveRecord Adapter</a> for GIS operations and types on PostgreSQL. In table <code>elements</code> I have attribute <code>:coordinates</code> type <code>geometry</code>. In ruby I can do coordinates.x and it returns x coordinate. But how can I parse this in serializer?</p>
  
  <pre><code>class ElementSerializer &lt; ActiveModel::Serializer
    attributes :coordinates
  end
  </code></pre>
  
  <p>In development returns: </p>
  
  <pre><code>"POINT (45.815927 16.00538)"
  </code></pre>
  
  <p>In production returns:</p>
  
  <pre><code>0101000000581CCEFC6AE8464003E962D34A013040
  </code></pre>
  
  <p>I want something like:</p>
  
  <pre><code>class ElementSerializer &lt; ActiveModel::Serializer
        attributes {:coordinates =&gt; [:coordinates.x, :coordinates.y]}
  end
  </code></pre>
  
  <p>Want to return:
  <code>[45.815927 16.00538]</code></p>