Product Service Get By UPC doesn't work

`ProductService::getByUpc()` expects a single entity like the `getById()` call but it receives a collection. 

This solves the issue:

```
/**
 * {@inheritdoc}
 */
public function getByUpc($upc)
{
        $this->guardString($upc);
        
        $collection = $this->getEntityCollection('items', ['upc' => $upc], self::COLLECTION_KEY);
        return $collection->getFirst();
}
```