title: ActiveAdmin Unpermitted Parameters despite being whitelisted
body: <p>I've whitelisted the params like so: </p>
  
  <pre><code>permit_params(%i{identity os current_ip clients site_ids sites domains})
  </code></pre>
  
  <p>but I get the following in the log:</p>
  
  <pre><code>{"utf8"=&gt;"✓", "_method"=&gt;"patch", "authenticity_token"=&gt;"aU8vwoMbWcMikSQLSGuVo5XTF+sAopRi0+OhgxXYvSKD9UbLaImB3k+uhf03PpWNiwtrZrEv0wVpT9pIcCkyag==", "server"=&gt;{"identity"=&gt;"", "os"=&gt;"UBUNTU 14.04", "current_ip"=&gt;"123.245.23.24", "site_ids"=&gt;["", "1"]}, "commit"=&gt;"Update Server", "controller"=&gt;"admin/servers", "action"=&gt;"update", "id"=&gt;"1"}
    Server Load (0.2ms)  SELECT  "servers".* FROM "servers" WHERE "servers"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
  Unpermitted parameters: :site_ids
  </code></pre>
  
  <p>My form looks like:</p>
  
  <pre><code>  form do |f|
      f.inputs "Server Details" do
        f.input :identity
        f.input :os, label: 'OS'
        f.input :current_ip, as: :string
        f.input :sites, as: :select, collection: Site.all
      end
      f.actions
    end
  </code></pre>
  
  <p>Why are params not being allowed despite being whitelisted?</p>