Strange behaviour with response.body in rspec controller tests

image_controller_spec.rb

# encoding: UTF-8
require "spec_helper"

describe Api::V1::ImagesController do
  context "on GET to :index" do
    before do
      FactoryGirl.create_list(Image, 2)
    end
     
    it "should find the strange rabl behaviour" do
      get :index, :format => :json
      response_images = JSON.parse(response.body)
      response_images.should == [{"image"=>{}}]
    end
  end
end

images_controller.rb

class Api::V1::ImagesController < Api::V1::BaseController
  def index
    respond_with(@images = Image.all)
  end
end

index.json.rabl

collection @images

extends 'api/v1/images/show'

show.json.rabl

object @image
attributes :id,
           :file_width,
           :file_height,
           :file_file_size,
           :file_content_type,
           :file_resolution,
           :file_colorspace,
           :file_depth,
           :file_aspect_ratio,
           :file_chromaticity,
           :file_created_at,
           :description,
           :tags,
           :campaigns,
           :origin_reference,
           :licence_note,
           :licence_expiration_at,
           :last_used_at,
           :created_at,
           :updated_at

node :file_name do |i|
  i.filename
end

node :status do |i|
  i.status == 0 ? I18n.t("images_edit.statuses.inactive") : I18n.t("images_edit.statuses.active")
end

node :client, :if => lambda { |i| i.client.present? } do |i|
  i.client.name
end

node :licencetype do |i|
  i.licencetype.present? ? I18n.t("licencetype.licencetypes.#{i.licencetype.name}") : nil
end

node :origin do |i|
  i.origin.present? ? I18n.t("origin.origins.#{i.origin.name}") : nil
end

node :category do |i|
  I18n.t("image_categories.#{i.category}")
end

node(:user_id, :if => lambda { |i| i.category == 'user_images' }) do |i|
  i.user_id
end

node(:image_type, :if => lambda { |i| i.category == 'user_images' }) do |i|
  i.image_type.present? ? i.image_type.name : nil
end

node :thumb_url do |i|
  api_v1_thumb_image_download_url(i)
end

node :preview_url do |i|
  api_v1_preview_image_download_url(i)
end

node :original_url do |i|
  api_v1_original_image_download_url(i)
end

node :original_fs_path do |i|
  i.file.current_path
end