Fixed #22085 -- Added a feature for setting non-expiring keys as the default.

This feature allows the default `TIMEOUT` Cache argument to be set to `None`,
so that cache instances can set a non-expiring key as the default,
instead of using the default value of 5 minutes.

Previously, this was possible only by passing `None` as an argument to
the set() method of objects of type `BaseCache` (and subtypes).