title: Rails Activeadmin : Check boxes values are not saved
body: <p>Model</p>
  
  <pre><code> # certification.rb
  
  class Certification &lt; ActiveRecord::Base
  
   extend Enumerize
   enumerize :certification_type, in: [:SEO, :CRM]
  
  end
  </code></pre>
  
  <p>My admin file</p>
  
  <pre><code> # admin/certification.rb
  
  ActiveAdmin.register Certification do
     permit_params :name,
                   :certification_type,
  
    form :html =&gt; { :enctype =&gt; "multipart/form-data" } do |f|
         f.inputs "Certifications" do
           f.input :name, :label =&gt; 'Nom'
           f.input :certification_type, :label =&gt; 'Type',
                                      as: :check_boxes
        end
       f.actions
    end
  end
  </code></pre>
  
  <p>The problem is with the <code>certification_type</code> field.
  When I tick one type in my activeadmin page, the entry isn't saved in the database. But when I change <code>as: :check_boxes</code> with a <code>as: :select</code>, it works.</p>
  
  <p>Do you know if there is a reason ? </p>
  
  <p>Thank you</p>