title: ActiveAdmin - how can I strip the filter / search terms
body: <p>In ActiveAdmin, how can I strip the search term? (remove whitespace around " steve@example.com ")</p>
  
  <p>I am trying to apply this to an email field.</p>
  
  <pre><code>ActiveAdmin.register User do
    ...
    filter :email, # &lt;- I want to strip! this search term
    filter :name
    filter :address_phone
    ...
  </code></pre>
  
  <p>Thanks</p>