max-age settings forces Set-Cookie parameter

With patch https://github.com/rack/rack/commit/74e0acd36353cf619bf56fce17bcb9ef1ab30547, there is ability on rack to set max-age parameter. Which is fine. Unfortunately this parameter is sent to browser only when the session cookie is created. Which is afaik wrong. You usually want to refresh the cookie with each page request. And setting 'expires' parameter behaves in this way as well. So this patch doest:

1) When max-age parameter is present, the Set-Cookie HTTP header is sent with each request
2) When both max-age and expires parameter are set, max-age has precedence
3) Added max-age conversion to string where aplicable, because user might not want to use string for "time interval"
On the last line, there is `.to_s` conversion. `max_age` should not be string so the conversion is needed for the string concatenation.

Other lines are indented just to follow some vertical alignment.