title: Restkit: RKValueTransformer transformationBlock doesn't called
body: <p>Here is what I have:</p>
  
  <p>in my JSON response I have an array of options:
  <code>Options : ["red","green", "blue"]</code></p>
  
  <p>I want to convert it to an string: <code>options = @"red, green, blue";</code> (I have defined <code>options</code> property of menuComponent entity as 'NSString')</p>
  
  <p>I have provided a customValueTransformer for that property, but transformationBlock doesn't get called.
  does anyone have any idea what am I doing wrong?</p>
  
  <pre><code>RKEntityMapping *menuComponentMapping = [RKEntityMapping mappingForEntityForName:@"MenuComponent" inManagedObjectStore:managedObjectStore];
  
  menuComponentMapping.identificationAttributes = @[@"menuComponentID"];
  [menuComponentMapping addAttributeMappingsFromDictionary:@{
                                                             @"ID":@"menuComponentID",
                                                             @"Name":@"name",
                                                             @"Description":@"descriptions",
                                                             @"Thumbnail":@"thumbnail"
                                                             }];
  //every menu component have some options
  RKAttributeMapping *optionsMapping = [RKAttributeMapping attributeMappingFromKeyPath:@"Options" toKeyPath:@"options"];
  
  //converting options to string
  RKValueTransformer *arrayToStringTransformer = [RKBlockValueTransformer valueTransformerWithValidationBlock:^BOOL(__unsafe_unretained Class sourceClass, __unsafe_unretained Class destinationClass) {
      // I don't know check what with what!
      return ([sourceClass isSubclassOfClass:[NSNumber class]] &amp;&amp; [destinationClass isSubclassOfClass:[NSString class]]);
  } transformationBlock:^BOOL(id inputValue, __autoreleasing id *outputValue, Class outputValueClass, NSError *__autoreleasing *error) {
      // Validate the input and output
  
      // Perform the transformation
  // iterating through options and append to a string
  
      return YES;
  }];
  
  //adding custom value transformer
  optionsMapping.valueTransformer = arrayToStringTransformer;
  //adding optionsMapping
  [menuComponentMapping addPropertyMapping:optionsMapping];
  [[RKValueTransformer defaultValueTransformer] insertValueTransformer:arrayToStringTransformer atIndex:0];
  
  RKRelationshipMapping *menuComponentRelationShip = [RKRelationshipMapping relationshipMappingFromKeyPath:@"MenuComponents" toKeyPath:@"children" withMapping:menuComponentMapping];
  
  //adding menucomponent children relationship to menuComponent
  [menuComponentMapping addPropertyMapping:menuComponentRelationShip];
  
  return menuComponentMapping;
  }
  </code></pre>