set _cache_open func even if not loading cache, fixes autoreload

The _cache_open attribute of the FileDataCacher class was not set when
settings[load_policy_key] was not True, so saving later failed.
As a precaution, replaced the `if ...: return` style with a plain
if structure to prevent such readability issues and added tests.
Merge pull request #1318 from smartass101/fix_cacher_open_func

set _cache_open func even if not loading cache, fixes autoreload