Optional PO number

[springimport]

Steps to reproduce

1. Add items to cart, go to checkout and select "Purchase Order".
   
2. Use REST API [POST] /V1/carts/mine/payment-information

{
  "paymentMethod": {
      "method": "purchaseorder"
   }
}

Expected result

Error: PO number is required.

Actual result

"90"

(number of new order)

Additional info

API documentation checkoutPaymentInformationManagementV1

quote-data-payment-interface {
  po_number (string, optional): Purchase order number ,
  ...
}

Q	A
Magento version	2.1.0 with sample data
PHP version	7 x64
Operating system	Windows 10 x64

[vityakopin]

Magento does not support your operating system. Please confirm that the issue reproduces on some supported system as well. Please see Magento technology stack requirements for more information.

[smoskaluk]

Internal ticket created MAGETWO-70558

[magento-engcom-team]

@springimport, thank you for your report.
We've created internal ticket(s) MAGETWO-70558 to track progress on the issue.