title: activeadmin carrierwave image hint not displayed
body: <p>HI I am using active admin with carrier gem. I am unable to view preview on image upload in hint
  my image src is always empty i.e <code>f.template.image_tag(f.object.image.url)</code> is empty</p>
  
  <p>I can see image relative url after clicking upload button.</p>
  
  <p>my form looks like this</p>
  
  <pre><code>ActiveAdmin.register Product do
  
    permit_params :name, :description , :category_id , :image
    form(:html =&gt; { :multipart =&gt; true }) do |f|
      f.inputs "Product" do
        f.input :category_id , :as =&gt; :select , :collection =&gt; Category.all
        f.input :name
        f.input :description
        f.input :image, :as =&gt; :file, :hint =&gt; f.template.image_tag(f.object.image.url) 
      end
      f.actions
    end
  end
  </code></pre>
  
  <p><em>model</em></p>
  
  <pre><code>class Product &lt; ActiveRecord::Base
      belongs_to :category
      mount_uploader :image, ImageUploader
  end
  </code></pre>
  
  <p><em>serializer</em></p>
  
  <pre><code>class ProductSerializer &lt; ActiveModel::Serializer
    attributes :id, :name, :description , :image
  end
  </code></pre>
  
  <p>my uploader looks like this</p>
  
  <pre><code>class ImageUploader &lt; CarrierWave::Uploader::Base
    storage :file
    def store_dir
      "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
    end
  end
  </code></pre>
  
  <p>GENERATE HTML</p>
  
  <pre><code>&lt;label class="label" for="product_image"&gt;Image&lt;/label&gt;
  &lt;input id="product_image" name="product[image]" type="file"&gt;
  &lt;p class="inline-hints"&gt;&lt;img src=""&gt;&lt;/p&gt;
  </code></pre>