Bullet fails when response body is Rack::File, issue #62

Implemented kikito's fix:

    # lib/bullet/rack.rb
    def empty?(response)
      # response may be ["Not Found"], ["Move Permanently"], etc.
      (response.is_a?(Array) && response.size <= 1) ||
        !response.respond_to?(:body) || 
        !response.body.respond_to?(:empty?) ||
        response.body.empty?
    end