best_before = None triggers function call everytime #11

#10 makes is_stale default to True when best_before is None. This prompts a function call every time someone calls the function.

IMO, this should not be the default behaviour, but rather an explicit choice.

A different approach would be exposing is_stale instead of best_before and letting users decide how it will be implemented (and not forcing the usage of timestamp, for example.).
Or making a best_before = infinite as default.

Ultimately, it is a decision for the maintainer @ecarrara.