Update PaymentIntent+API.swift

[dasportillo]

Fix: PaymentIntent Confirmation with Client Secret

Summary

This change addresses an issue with completing the payment intent in Stripe. The modification ensures that the client_secret is handled correctly during the transaction confirmation at the /paymentintent/{id}/confirm endpoint.

Motivation

This change fixes a bug where the Stripe API did not accept the client_secret in the same request during the transaction confirmation. Without this fix, the payment intent could not be completed successfully.

Example code demonstrating the issue:

# Example code showing the issue
response = stripe.PaymentIntent.confirm(
  "pi_123456789",
  {
    "client_secret": "sk_test_4eC39HqLyjWDarjtT1zdp7dc"
  }
)

[CLAassistant]

All committers have signed the CLA.