Restore logging if no completion handler given

[richardbuckle]

The logging in NSManagedObjectContext+DCTCoreDataStack.m was never called because the override in _DCTCDSManagedObjectContext was adding a no-op completion handler if none was passed in.

Moving the logging to the completion handler added by _DCTCDSManagedObjectContext fixes this.

(Fix suggested by Daniel Tull)