dispatch_barrier_async can lead to inconsistent values when using mutable objects

dispatch_barrier_async works great for this project, except for the following situation:
1. NSMutableDictionary *registeredDefaults = [[NSMutableDictionary alloc] init];
2. [[NSUserDefaults standardUserDefaults] registerDefaults:registeredDefaults];
3. [registeredDefaults setObject:@"FOO" forKey:@"BAR"];
4. NSObject *value = [[NSUserDefaults standardUserDefaults] objectForKey:@"BAR"];
- With sim performance ON, value may be equal to nil or equal to @"FOO" depending on if line 3 executed before the internal barrier_async block executed.
- With sim performance OFF, value is always equal to nil. Mutations after line 2 are not acknowledged.

The code above is incorrect anyways (you wouldn't expect it to work) -- but we still need to honor the behavior of NSUserDefaults. You could emulate this bug using -setObject:forKey: with a mutable object.