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.
Merge pull request #24 from rhiller/master

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