cache.get_or_set won't cache None results

[Phill Tornroth]

get_or_set docstring says "If the key does not exist, add the key and set it to the default value." -- but that's not quite what it does. It will perform a set if the key doesn't exist, or if the cached value is None.

I think in order to be doing what it says on the tin it'd need to be:

if self.has_key(key, version=version):
        return self.get(key, version=version)
else:
    if callable(default):
        default = default()
    if default is not None:
        self.add(key, default, timeout=timeout, version=version)
        # Fetch the value again to avoid a race condition if another
        # caller added a value between the first get() and the add()
        # above.
        return self.get(key, default, version=version)

I'd find this useful in cases where None was an expensive result to arrive at. If there's spiritual alignment with the suggestion, I'm happy to prepare and submit a change with tests.