title: Polymorphic Association in Active Admin Not Passing All Parameters
body: <p>I created a Polymorphic Association in Active Admin for the creation of Keyword associated with Industry. I can display the association fine in Active Admin (the association was created in the console) but when I Update and create New, only the keyword parameters gets passed. If I go back to the console and update, the updated data is properly displayed in Active Admin.</p>
  
  <p>I never get an error.</p>
  
  <p>I have:</p>
  
  <pre><code>class Industry &lt; ActiveRecord::Base
      attr_accessible :name, :keywords, :keywords_attributes
  
    # Associations
    has_many :profiles
    has_many :companies
    has_many :users
    has_many :keywords, as: :keyable
    accepts_nested_attributes_for :keywords
  end
  
  class Keyword &lt; ActiveRecord::Base
  
    attr_accessible :name, :profile_id, :active,
                    :keyable_attributes, :rating,
                    :keyable_id, :keyable_industry
  
    # Associations
    belongs_to :profile # FK
    belongs_to :keyable, polymorphic: true
    accepts_nested_attributes_for :keyable
  
    attr_accessor :keyable_industry
  
    def keyable_industry
      self.keyable.id if self.keyable.is_a? Industry
    end
  end
  
  
  ActiveAdmin.register Keyword do
  index do
      column :name
      column :active
      column :rating
      column "Keyword Group", :keyable
      column :keyable_type
      default_actions
  end
  form do |f|
      f.inputs "Conference Detail" do
          f.input :name
          f.input :active
          f.input :rating
          f.input :keyable_industry
      end
  
      f.inputs "Industry" do
          f.input :keyable_industry, label: "Industry", 
          :as =&gt; :select,
          :collection =&gt; Industry.all.map {|i| [i.name]},
          :include_blank =&gt; false
      end
      f.actions
    end
   end
  
  Development Log Output
  Started PUT "/admin/keywords/2" for 127.0.0.1 at 2013-09-13 11:41:32 -0400
  Processing by Admin::KeywordsController#update as HTML
  Parameters: {"utf8"=&gt;"✓",    "authenticity_token"=&gt;"nnbMIHy1YndWNOKyF+LSABYyeQMpKQAiTyqGbL2sq3g=", "keyword"=&gt;  {"name"=&gt;"test", "active"=&gt;"1", "rating"=&gt;"1", "keyable_industry"=&gt;"Agriculture &amp;   Forestry"}, "commit"=&gt;"Update Keyword", "id"=&gt;"2"}
  User Load (1.1ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 1 LIMIT 1
  Keyword Load (0.7ms)  SELECT "keywords".* FROM "keywords" WHERE "keywords"."id" = $1   LIMIT 1  [["id", "2"]]
   (0.7ms)  BEGIN
   (1.1ms)  UPDATE "keywords" SET "name" = 'test', "updated_at" = '2013-09-13   15:41:32.287147' WHERE "keywords"."id" = 2
   (2.9ms)  COMMIT
   Redirected to http://blog.dev/admin/keywords/2
   Completed 302 Found in 14ms (ActiveRecord: 0.0ms)
  </code></pre>