Change type of _sourceObject ivars to match the property which is alised to them.

With unmatched type between property and storage, lldb gets confused when printing the object.

Error is: instance method ''sourceObject' has incompatible result types in different translation units ('NSObject *' vs. 'id')

Signed-off-by: Roger Nolan <rog@hatbat.net>

Code/ObjectMapping/RKObjectLoader.h

@@ -136,8 +136,8 @@ typedef void(^RKObjectLoaderDidLoadObjectsDictionaryBlock)(NSDictionary *diction
  * includes Core Data specific mapping logic.
  */
 @interface RKObjectLoader : RKRequest {
-    id _sourceObject;
-    id _targetObject;
+    NSObject* _sourceObject;
+    NSObject* _targetObject;
     dispatch_queue_t _mappingQueue;
 }