Fixes nested array for used products cache key

When $requiredAttributeIds is passed as an array, it was previously being added to the $keyParts array directly, which resulted in a nested array. When this nested array is passed to getUsedProductsCacheKey(), the implode() in that function throws a notice and produces a cache key that contains the non-unique string "Array" as a part. This can result in caching errors where simple children of configurable products are concerned (e.g., swatches).

app/code/Magento/ConfigurableProduct/Model/Product/Type/Configurable.php

@@ -1232,6 +1232,8 @@ public function isPossibleBuyFromList($product)
     /**
      * Returns array of sub-products for specified configurable product
      *
+     * $requiredAttributeIds - one dimensional array, if provided
+     *
      * Result array contains all children for specified configurable product
      *
      * @param  \Magento\Catalog\Model\Product $product
@@ -1245,9 +1247,12 @@ public function getUsedProducts($product, $requiredAttributeIds = null)
             __METHOD__,
             $product->getData($metadata->getLinkField()),
             $product->getStoreId(),
-            $this->getCustomerSession()->getCustomerGroupId(),
-            $requiredAttributeIds
+            $this->getCustomerSession()->getCustomerGroupId()
         ];
+        if (!is_null($requiredAttributeIds)) {
+            sort($requiredAttributeIds);
+            $keyParts[] = implode('', $requiredAttributeIds);
+        }
         $cacheKey = $this->getUsedProductsCacheKey($keyParts);
         return $this->loadUsedProducts($product, $cacheKey);
     }