Issue #290 gives an example of a wiki with protected pages whose
protection definitions have no 'expiry' key at all. This seems
strange and, on a quick through the mediawiki code, difficult
to achieve, but since there's a live site out there that does it,
and mediawiki *does* seem to have at least some code to handle
such cases (e.g. how the expiry shows as 'indefinite' in the
page information), let's handle it too, by representing this as
None.

Signed-off-by: Adam Williamson <awilliam@redhat.com>