Update paypalplatform.php

Modifying the global variable $API_EndPoint causes corruption should various calls be made to hashcall during the lifetime of the module. Example as follows:

On the first call to hashcall from CallPaymentDetails $API_Endpoint is: https://svcs.sandbox.paypal.com/AdaptivePayments/PaymentDetails

On the second call to hashcall from CallPay $API_Endpoint is: https://svcs.sandbox.paypal.com/AdaptivePayments/PaymentDetails/Pay

Solution: Preserve variable $API_Endpoint across calls.