Auth capture idempotency and refund idempotency (#143)

* Idempotency for capture auth

authorization.go

@@ -25,13 +25,28 @@ func (c *Client) GetAuthorization(authID string) (*Authorization, error) {
 // To use this method, the original payment must have Intent set to "authorize"
 // Endpoint: POST /v2/payments/authorizations/ID/capture
 func (c *Client) CaptureAuthorization(authID string, paymentCaptureRequest *PaymentCaptureRequest) (*PaymentCaptureResponse, error) {
+	return c.CaptureAuthorizationWithPaypalRequestId(authID, paymentCaptureRequest, "")
+}
+
+// CaptureAuthorization captures and process an existing authorization with idempotency.
+// To use this method, the original payment must have Intent set to "authorize"
+// Endpoint: POST /v2/payments/authorizations/ID/capture
+func (c *Client) CaptureAuthorizationWithPaypalRequestId(
+	authID string,
+	paymentCaptureRequest *PaymentCaptureRequest,
+	requestID string,
+) (*PaymentCaptureResponse, error) {
 	req, err := c.NewRequest("POST", fmt.Sprintf("%s%s", c.APIBase, "/v2/payments/authorizations/"+authID+"/capture"), paymentCaptureRequest)
 	paymentCaptureResponse := &PaymentCaptureResponse{}
 
 	if err != nil {
 		return paymentCaptureResponse, err
 	}
 
+	if requestID != "" {
+		req.Header.Set("PayPal-Request-Id", requestID)
+	}
+
 	err = c.SendWithAuth(req, paymentCaptureResponse)
 	return paymentCaptureResponse, err
 }

order.go

@@ -113,13 +113,27 @@ func (c *Client) CaptureOrderWithPaypalRequestId(
 // RefundCapture - https://developer.paypal.com/docs/api/payments/v2/#captures_refund
 // Endpoint: POST /v2/payments/captures/ID/refund
 func (c *Client) RefundCapture(captureID string, refundCaptureRequest RefundCaptureRequest) (*RefundResponse, error) {
+	return c.RefundCaptureWithPaypalRequestId(captureID, refundCaptureRequest, "")
+}
+
+// RefundCapture with idempotency - https://developer.paypal.com/docs/api/payments/v2/#captures_refund
+// Endpoint: POST /v2/payments/captures/ID/refund
+func (c *Client) RefundCaptureWithPaypalRequestId(
+	captureID string,
+	refundCaptureRequest RefundCaptureRequest,
+	requestID string,
+) (*RefundResponse, error) {
 	refund := &RefundResponse{}
 
 	req, err := c.NewRequest("POST", fmt.Sprintf("%s%s", c.APIBase, "/v2/payments/captures/"+captureID+"/refund"), refundCaptureRequest)
 	if err != nil {
 		return refund, err
 	}
 
+	if requestID != "" {
+		req.Header.Set("PayPal-Request-Id", requestID)
+	}
+
 	if err = c.SendWithAuth(req, refund); err != nil {
 		return refund, err
 	}