Filtered section info retains its source info

@joe-goullaud 
There is a bug I missed that seems like it could be fairly common. Fetched collection list generates its section info on-demand, so if a filtered list is observing a fetched list, the weak source section info property would go to nil basically immediately, resulting in an apparent zero-object section in the final list.

I can't think of a case where having this strong would cause a retain cycle, so I think this is safe.