title: ActiveAdmin create a custom filter based on getter
body: <p>I have an ActiveAdmin page with filters.
  I would like to set a custom filter based on a getter (a method defined on the resource, not an ActiveRecord attribute).</p>
  
  <pre><code>class House
    def is_enchanted?
      # Ask to witches and ghosts...
    end
  end
  </code></pre>
  
  <p>This is the ActiveAdmin resource:</p>
  
  <pre><code>ActiveAdmin.register House do
    filter :is_enchanted?
  end
  </code></pre>
  
  <p>The above code raises the following exception:</p>
  
  <pre><code>undefined method `is_enchanted?_eq' for #&lt;Ransack::Search:0xc188178&gt;
  </code></pre>