fixes crash in cocos2dx-store

when converting a MarketItem into a dictionary, the product id is not only set for the key "IOS_ID" but also for the "PRODUCT_ID" key;
fixes crash in cocos2dx-store, which tries to read the product id with the "PRODUCT_ID" key (which returned NULL):
In CCSoomla::easyNDKCallback(...), with the methodname "CCEventHandler::onMarketItemsRefreshed", the productId is read like this:
__String *productId = dynamic_cast<__String *>(marketItem->objectForKey("productId"));
But the productId was never set when the dictionary was created which lead to a crash when calling "productId->getCString()"