Allow a nil starting value for -aggregateWithStart:reduce:

Fixes #1007.
Merge pull request #1074 from ReactiveCocoa/allow-nil-aggregate-start

Allow a nil starting value for -aggregateWithStart:reduce:
Merge remote-tracking branch 'origin/3.0-development' into mutations
Tests for unordered mutations
Tests for ordered mutations
Mutations aren't sent for keys without to-many accessors