title: createDestinationInstancesForSourceInstance not called
body: <p>I am trying to do custom migration for CoreData entry. I need to change name and type for an attribute.</p>
  
  <p>So I added new datamodel and mappingmodel. For example, I have myData(String) in old model and I need CarDate(Date) in new model.
  I added row: CarDate => $source.myData to the *.xcmappingmodel file and created class:</p>
  
  <pre><code>@implementation RemindersMigration
  
  - (BOOL)createDestinationInstancesForSourceInstance:(NSManagedObject *)sInstance
                                    entityMapping:(NSEntityMapping *)mapping
                                          manager:(NSMigrationManager *)manager
                                            error:(NSError **)error {
  
  NSManagedObject *newObject =
  [NSEntityDescription insertNewObjectForEntityForName:[mapping destinationEntityName]
                                inManagedObjectContext:[manager destinationContext]];
  
  NSString *dateString = [sInstance valueForKey:@"myData"];
  
  NSDateFormatter *formatter = ...;
  
  NSDate *date = [formatter dateFromString:dateString];
  
  [newObject setValue:date forKey:@"CarDate"];
  
  [manager associateSourceInstance:sInstance withDestinationInstance:newObject forEntityMapping:mapping];
  
  return YES;
  </code></pre>
  
  <p>}</p>
  
  <p>I tested migration with simulator, so I switched to old version with Git, added new entry, switched to new version. In CarDate I have nil.
  Please point me where I am wrong?</p>