Make view specs fail when image_tag is used for a non-existing asset

spec_helper_snippet.rb

  config.before(:each) do
    if self.class.metadata[:type] == :view
      view.singleton_class.send :alias_method, :original_image_tag, :image_tag
      view.singleton_class.send :define_method, :image_tag do |path, options = {}|
        normalized_path = path_to_image(path.to_s)

        asset_paths = Sprockets::Helpers::RailsHelper::AssetPaths.new(nil) # hack: easy access to is_uri?
        unless asset_paths.is_uri?(normalized_path)
          normalized_path = normalized_path[asset_prefix.length+1..-1]
          unless Rails.application.assets.find_asset(normalized_path) or path.respond_to?(:stubbed?)
            message = "image_tag called for non-existing asset '#{path}'.\n" \
              "Make its first argument respond to :stubbed? if it is a stubbed path"
            raise message
          end
        end
        original_image_tag path, options
      end
    end
  end