MC-34431: Set Billing Address API defects

- fixed
- modified test

Author: Oleksandr Dubovyk

app/code/Magento/Quote/Model/GuestCart/GuestBillingAddressManagement.php

@@ -46,7 +46,7 @@ public function assign($cartId, \Magento\Quote\Api\Data\AddressInterface $addres
     {
         /** @var $quoteIdMask QuoteIdMask */
         $quoteIdMask = $this->quoteIdMaskFactory->create()->load($cartId, 'masked_id');
-        return $this->billingAddressManagement->assign($quoteIdMask->getQuoteId(), $address, $useForShipping);
+        return (int)$this->billingAddressManagement->assign($quoteIdMask->getQuoteId(), $address, $useForShipping);
     }
 
     /**

app/code/Magento/Quote/Test/Unit/Model/GuestCart/GuestBillingAddressManagementTest.php

@@ -97,6 +97,8 @@ public function testAssign()
     {
         $addressId = 1;
         $this->billingAddressManagementMock->expects($this->once())->method('assign')->willReturn($addressId);
-        $this->assertEquals($addressId, $this->model->assign($this->maskedCartId, $this->addressMock));
+        $actualAddressId = $this->model->assign($this->maskedCartId, $this->addressMock);
+        $this->assertIsInt($actualAddressId);
+        $this->assertEquals($addressId, $actualAddressId);
     }
 }