title: primaryKeyAttribute not working Restkit/Core Data
body: <p>I just installed the framework restkit 0.9.3 and followed the Discussion Board example. Well, everything just worked great, however when I tried to use Core Data my User NSManagedObject class is duplicating even after declaring his primaryKeyAttribute (userID). For example, when I send a login request to my web-server, I return <code>{"user":{"id":1, "username":"teste", ...}}</code> .. but it seems to create a new row every time it invoques objectLoader:didLoadObjects.</p>
  
  <p><em>User table:</em></p>
  
  <p><img src="https://i.stack.imgur.com/iPdZO.png" alt="enter image description here"></p>
  
  <p><em>Example code:</em></p>
  
  <p><strong>~ AppDelegate.m didFinishLaunching</strong></p>
  
  <pre><code>RKManagedObjectMapping* userMapping = [RKManagedObjectMapping mappingForClass:[User class]];    
  userMapping.primaryKeyAttribute = @"userID";
  userMapping.setDefaultValueForMissingAttributes = YES; // clear out any missing attributes (token on logout)
  [userMapping mapKeyPathsToAttributes:
       @"id", @"userID",
       @"email", @"email",
       @"username", @"username",
       @"password", @"password",
       nil];
  
  [objectManager.mappingProvider registerMapping:userMapping withRootKeyPath:@"user"];
  </code></pre>
  
  <p><strong>~ User.m loginWithDelegate</strong></p>
  
  <pre><code>- (void)loginWithDelegate:(NSObject&lt;UserAuthenticationDelegate&gt;*)delegate {
      _delegate = delegate;   
      [[RKObjectManager sharedManager] postObject:self delegate:self block:^(RKObjectLoader* loader) {
          loader.resourcePath = @"/login";
          loader.serializationMapping = [RKObjectMapping serializationMappingWithBlock:^(RKObjectMapping* mapping) {
              [mapping mapAttributes:@"username", @"password", nil];            
          }];
      }];
  }
  </code></pre>
  
  <p><strong>~ User.m didLoadObjects (RKObjectLoaderDelegate)</strong></p>
  
  <pre><code>- (void)objectLoader:(RKObjectLoader*)objectLoader didLoadObjects:(NSArray *)objects {
  
      if ([objectLoader wasSentToResourcePath:@"/login"]) {
          [self loginWasSuccessful];
      }
  
      NSLog(@"number of user rows: %i", [User findAll].count);
  }
  </code></pre>
  
  <p>What am I doing wrong?</p>