Cache doesn't work in _query_dns

[jlaundry]

Currently, _query_dns uses the following logic to see if a cache should be used:

checkdmarc/checkdmarc.py

Lines 637 to 640 in 7cc07db

	if cache is None:
	cache = DNS_CACHE
	if cache:
	records = cache.get(cache_key, None)

However, if cache doesn't return True if cache is an ExpiringDict, so the cache is never used.