title: SKProductsRequest deallocated, even though using ARC?
body: <p>I'm trying to set up an SKProductsRequest, but I continually get the error EXC_BAD_ACCESS. I know it has to do with ARC.
  In my .h file, I have SKProductsRequestDelegate.</p>
  
  <p>These are the main functions in my .m file:</p>
  
  <pre><code>- (void)requestProUpgradeProductData {
      NSSet *productIdentifiers = [NSSet setWithObject:kInAppPurchaseProUpgradeProductId];
      productsRequest = [[SKProductsRequest alloc] initWithProductIdentifiers:productIdentifiers];
      productsRequest.delegate = self;
      [productsRequest start];
  }
  
  #pragma mark -
  #pragma mark SKProductsRequestDelegate methods
  
  - (void)productsRequest:(SKProductsRequest *)request didReceiveResponse:(SKProductsResponse *)response {
      NSArray *products = response.products;
      //proUpgradeProduct = [products count] == 1 ? [products firstObject] : nil;
      proUpgradeProduct = [products objectAtIndex:0];
      if (proUpgradeProduct) {
          NSLog(@"Product title: %@" , proUpgradeProduct.localizedTitle);
          NSLog(@"Product description: %@" , proUpgradeProduct.localizedDescription);
          NSLog(@"Product price: %@" , proUpgradeProduct.price);
          NSLog(@"Product id: %@" , proUpgradeProduct.productIdentifier);
      }
  
      for (NSString *invalidProductId in response.invalidProductIdentifiers) {
          NSLog(@"Invalid product id: %@" , invalidProductId);
      }
  
      // finally release the reqest we alloc/init’ed in requestProUpgradeProductData
      productsRequest = nil;
  
      [self purchaseProUpgrade];
  
      [[NSNotificationCenter defaultCenter] postNotificationName:kInAppPurchaseManagerProductsFetchedNotification object:self userInfo:nil];
  }
  </code></pre>
  
  <p>When I enabled NSZombieEnabled, then this is what I get: <code>"-[InAppPurchaseManager respondsToSelector:]: message sent to deallocated instance."</code></p>
  
  <p>Any help would be greatly appreciated.
  Thanks!</p>