Crash with RestKit when starting requests in quick succession
===

I have a button with two buttons which start the download of some data from a web server with [RestKit](http://restkit.org). 

Now if the user taps the two buttons repeatedly in quick succession my app crashes and produces the crash log below.


I initiate my requests like so:

    -(void)loadDataAtPath:(NSString *)path completion:(ResultArrayBlock)completionBlock failed:(ResultFailedBlock)failedBlock
    {
        RKObjectMapping *groupMapping = [Group mapping];
        
        [self.objectManager.mappingProvider setMapping:groupMapping forKeyPath:@"groups.group"];
        [self.objectManager.mappingProvider setObjectMapping:groupMapping forKeyPath:path];  
        
        [self.objectManager loadObjectsAtResourcePath:path usingBlock:^(RKObjectLoader *loader) {
            loader.onDidLoadObjects = ^(NSArray *array){
    
                // Do the reverse mapping of the group
                for (Group *c in array) {
                    for(Person *p in c.persons){
                        p.group = c;
                    }
                }
                
                completionBlock(array);
                
            };
            loader.onDidFailWithError = failedBlock;
        }];
    }

I first thought that the problem was the for-loop where I do some after-mapping setup of my data, but the problem still persists even when commenting the for-loop.

### The crash
    Application Specific Information:
    *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSCFDictionary: 0x3c14a0> was mutated while being enumerated.'
    
    Last Exception Backtrace:
    0   CoreFoundation                      0x3734e88f __exceptionPreprocess + 162
    1   libobjc.A.dylib                     0x35053259 objc_exception_throw + 32
    2   CoreFoundation                      0x3734e3b3 __NSFastEnumerationMutationHandler + 162
    3   MyApp                          		0x0008f5bf -[RKObjectMapper performKeyPathMappingUsingMappingDictionary:] + 407
    4   MyApp                        		0x0008fa45 -[RKObjectMapper performMapping] + 673
    5   MyApp                          		0x0008ac7d -[RKObjectLoader mapResponseWithMappingProvider:toObject:inContext:error:] + 1045
    6   MyApp                          		0x0008b04f -[RKObjectLoader performMapping:] + 575
    7   MyApp                          		0x0008b22b __47-[RKObjectLoader performMappingInDispatchQueue]_block_invoke_0 + 247
    8   libdispatch.dylib                   0x3046ec59 _dispatch_call_block_and_release + 12
    9   libdispatch.dylib                   0x30479cab _dispatch_queue_drain + 274
    10  libdispatch.dylib                   0x30479b19 _dispatch_queue_invoke$VARIANT$up + 36
    11  libdispatch.dylib                   0x3047a78b _dispatch_worker_thread2 + 214
    12  libsystem_c.dylib                   0x33bbddfb _pthread_wqthread + 294
    13  libsystem_c.dylib                   0x33bbdcd0 start_wqthread + 8


