Incorrect LIQPAY_API_CHECKOUT_URL

[AndriiStefaniv]

Overview

Looks like API was changed and now SDK points to wrong checkout URL.
Steps to reproduce:

1. Generate an HTML-form with cnb_form method for valid P2P request.
2. Open the HTML-form in a browser.
3. Push "Pay".
4. Result:

• Expected result: a payment LiqPay page should be rendered. Example:
  
• Actual result: an error page was rendered:
  

A cause is the invalid checkout URL:

String LIQPAY_API_CHECKOUT_URL = "https://www.liqpay.ua/api/3/checkout";

but that should be:

String LIQPAY_API_CHECKOUT_URL = "https://www.liqpay.ua/api/checkout";