use session_id at SimpleCaptcha::Utils::generate_key()

[nakajijapan]

It is wasteful not to use this code at default.

      def simple_captcha_key(key_name = nil)
        if key_name.nil?
          session[:captcha] ||= SimpleCaptcha::Utils.generate_key(session[:id].to_s, 'captcha')
        else
          SimpleCaptcha::Utils.generate_key(session[:id].to_s, key_name)
        end
      end 

I want to change session[:id] into session[:session_id].
Then, we can use session_id at default and simple_capthca_data.key become more unique.