Revert to mod_python 3.3.1 semantics where cookie expiration is sent to client
Revert to mod_python 3.3.1 semantics where cookie expiration is sent to client

Without the "expires" attribute in the _valid_attr list, cookie expirations are never returned by the __str__ method, and consequently are never sent to the browser.
dacaiguoguo/mod_python,runt18/mod_python,carlmcdade/mod_python,grisha/mod_python,runt18/mod_python,grisha/mod_python,runt18/mod_python,dacaiguoguo/mod_python,carlmcdade/mod_python,dacaiguoguo/mod_python,grisha/mod_python,carlmcdade/mod_python