Allow "uploads via url" with Rails and Paperclip

gistfile1.rb

#
# See also: http://almosteffortless.com/2008/12/11/easy-upload-via-url-with-paperclip/
#
# Allow "uploads via url" with Rails and Paperclip
# 
# http://www.thoughtbot.com/projects/paperclip
# http://github.com/thoughtbot/paperclip/tree/master
# http://groups.google.com/group/paperclip-plugin/browse_thread/thread/456401eb93135095
#
# This example is designed to work with a "Photo" model that has an "Image" attachment
# It requires adding a <?>_remote_url column for your attachment - "image_remote_url" in this case
#

# app/controllers/photos_controller.rb

class PhotosController < ApplicationController
  
  def create
    @photo = Photo.new(params[:photo])
    if @photo.save
      redirect_to photos_path
    else
      render :action => 'new'
    end
  end
  
end

# app/views/photos/new.html.erb

<%= error_messages_for :photo %>
<% form_for :photo, :html => { :multipart => true } do |f| %>
  Upload a photo: <%= f.file_field :image %><br>
  ...or provide a URL: <%= f.text_field :image_url %><br>
  <%= f.submit 'Submit' %>
<% end %>

# app/models/photo.rb

require 'open-uri'

class Photo < ActiveRecord::Base
  
  attr_accessor :image_url
  
  has_attached_file :image # etc...
  
  before_validation :download_remote_image, :if => :image_url_provided?

  validates_presence_of :image_remote_url, :if => :image_url_provided?, :message => 'is invalid or inaccessible'
    
  private
  
  def image_url_provided?
    !self.image_url.blank?
  end
  
  def download_remote_image
    io = open(URI.parse(image_url))
    io.original_filename = io.base_uri.path.split('/').last
    self.image = io
    self.image_remote_url = image_url
  rescue # catch url errors with validations instead of exceptions (Errno::ENOENT, OpenURI::HTTPError, etc...)
  end
  
end

This results in a stack overflow in the latest version of Paperclip (v3.4.1)