MAGETWO-36840: low_stock_date shows only year when requesting stockItems information of product through API service

 - update API functional test for SOAP

Author: Mike Weis

dev/tests/api-functional/testsuite/Magento/CatalogInventory/Api/StockItemTest.php

@@ -223,7 +223,7 @@ public function saveStockItemBySkuWithWrongInputDataProvider()
                     'enable_qty_increments' => '',
                     'use_config_manage_stock' => 1,
                     'manage_stock' => 1,
-                    'low_stock_date' => 0,
+                    'low_stock_date' => '',
                     'is_decimal_divided' => '',
                     'stock_status_changed_auto' => 0
                 ],