Page.php: when we cache, use null default

This saves resources on pages where the API call returns the present
default, e.g. array(), and yet we'll never use the cache.

It also avoids some odd logic errors, e.g. at present, AFAICT,
is_watched always returns false if $force has never been used.
Merge pull request #53 from Jarry1250/pageclass

Page.php: when we cache, use null default