Fix to MGProducts throwing an exception when fetching a single product v...

Fix to MGProducts throwing an exception (trace below) when fetching a single product via `makeProductWithId` and `fetchWithCompletion`.

MGProducts.m `(id)getModelFromResponse:(id)responseObject` method could receive two types of NSDictionaries with product information, for a single product request (via `makeProductWithId` and `fetchWithCompletion`) this is a simple hash of product keys and values, however for a collection (e.g. via `makeProductCollection` and `fetchWithCompletion`) this is a multidimensional hash of productId and product dictionary hash.  Only the latter type was supported.

```
2014-04-24 12:04:33.012 App[25631:60b] -[__NSCFString objectForKeyedSubscript:]: unrecognized selector sent to instance 0xb462a80
2014-04-24 12:04:33.015 App[25631:60b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString objectForKeyedSubscript:]: unrecognized selector sent to instance 0xb462a80'
*** First throw call stack:
(
  0   CoreFoundation                      0x026a21e4 __exceptionPreprocess + 180
  1   libobjc.A.dylib                     0x01d3c8e5 objc_exception_throw + 44
  2   CoreFoundation                      0x0273f243 -[NSObject(NSObject) doesNotRecognizeSelector:] + 275
  3   CoreFoundation                      0x0269250b ___forwarding___ + 1019
  4   CoreFoundation                      0x026920ee _CF_forwarding_prep_0 + 14
  5   App                                 0x0008c023 -[MGProducts mapResponseToModelObjectWithResponse:] + 111
  6   App                                 0x0008bec1 -[MGProducts getModelFromResponse:] + 480
  7   App                                 0x00096d71 __50-[RestApiMethod runWithParams:filters:completion:]_block_invoke + 48
  8   App                                 0x0005e727 __55-[AFHTTPSessionManager GET:parameters:success:failure:]_block_invoke + 94
  9   App                                 0x00073651 __72-[AFURLSessionManagerTaskDelegate URLSession:task:didCompleteWithError:]_block_invoke_288 + 156
  10  libdispatch.dylib                   0x02ccd7b8 _dispatch_call_block_and_release + 15
  11  libdispatch.dylib                   0x02ce24d0 _dispatch_client_callout + 14
  12  libdispatch.dylib                   0x02cd0726 _dispatch_main_queue_callback_4CF + 340
  13  CoreFoundation                      0x0270743e __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 14
  14  CoreFoundation                      0x026485cb __CFRunLoopRun + 1963
  15  CoreFoundation                      0x026479d3 CFRunLoopRunSpecific + 467
  16  CoreFoundation                      0x026477eb CFRunLoopRunInMode + 123
  17  GraphicsServices                    0x0445a5ee GSEventRunModal + 192
  18  GraphicsServices                    0x0445a42b GSEventRun + 104
  19  UIKit                               0x009fcf9b UIApplicationMain + 1225
  20  App                                 0x0001a49d main + 141
  21  libdyld.dylib                       0x02f17701 start + 1
)
```