Adds Invoice ID to DoCapture calls #PFW-222

[mgmartel]

Since July 26th Paypal has been refusing our captures of authorized Paypal Express payments with error code 10003 and the message: "Invoice ID is required".

Though the documentation still doesn't indicate that Invoice IDs are required on DoCapture calls, we found that adding the INVNUM parameter fixes the issue.

This PR looks for the invoice id prefix on the payment gateway and adds the INVNUM accordingly.

• I have only tested this with PayPal Express. Maybe this change should be limited to PPEC only?
• The invoice ID is generated on the fly. If the user changes the invoice ID prefix between authorizatino & capture, this fix will generate the wrong Invoice ID. Should the Invoice ID be stored as post meta?

[angelleye]

Thanks for the feedback on this. Rather than generate a random value we should probably just send the same value that would have been sent with the original authorization, which would be the Woo Order ID (WP Post ID). It would also include any invoice prefix value set in our options.

We'll make necessary adjustments here and get this included in our next update. Thanks again!