wsgi: Fix CORS cookie lifespan.

"expires" takes an absolute date, "max-age" takes a number of seconds until
expiration.
So switch to "max-age": according to Mozilla Developer Network, it is
supported by all major browsers, and by IE since version 8.

caucase/wsgi.py

@@ -688,7 +688,7 @@ class Application(object):
     """
     cookie = SimpleCookie({self._cors_cookie_id: value})[self._cors_cookie_id]
     cookie['path'] = environ.get('SCRIPT_NAME') or '/',
-    cookie['expires'] = A_YEAR_IN_SECONDS
+    cookie['max-age'] = A_YEAR_IN_SECONDS
     # No "secure" flag: cookie is not secret, and is protected against
     # tampering on client side.
     # No "httponly" flag: cookie is protected against tampering on client side,