Allow modification of the data in objectLoader:willMapData: by creating a mutable copy of the parsed data if the delegate method is implemented.

Code/ObjectMapping/RKObjectLoader.h

@@ -58,7 +58,8 @@
 /**
  Invoked just after parsing has completed, but before object mapping begins. This can be helpful
  to extract data from the parsed payload that is not object mapped, but is interesting for one
- reason or another.
+ reason or another. The mappableData will be made mutable via mutableCopy before the delegate
+ method is invoked.
  */
 - (void)objectLoader:(RKObjectLoader*)loader willMapData:(id)mappableData;
 

Code/ObjectMapping/RKObjectLoader.m

@@ -131,6 +131,7 @@
     }
     
     if ([self.delegate respondsToSelector:@selector(objectLoader:willMapData:)]) {
+        parsedData = [[parsedData mutableCopy] autorelease];
         [(NSObject<RKObjectLoaderDelegate>*)self.delegate objectLoader:self willMapData:parsedData];
     }