Cache functions returning None with decorators

[valo-joonas]

This is seemingly the same issue as #452 but as is noted in the comments in that issue, the skipping of None values is still hardcoded into the decorator @cached.

aiocache/aiocache/decorators.py

Lines 137 to 141 in ceb0b5e

	value = await self.get_from_cache(key)
	if value is not None:
	return value

[Dreamsorcerer]

Response is still the same though, PR with a failing test would be a great start.