Feature request: Allow retrieveUpcoming to use only the subscription

[reacuna]

According the the docs for version 2019-03-14 (maybe earlier, I'm not sure), the customer parameter is required only if the subscription parameter is not set.
The implementation however, always uses the first parameter as the customer id:

stripe-node/lib/resources/Invoices.js

Line 39 in 8b2c36c

var url = 'upcoming?customer=' + urlData.customerId;

Any chance to allow to use a subscription id without a customer?

[remi-stripe]

@reacuna Thanks a lot for the report. I had no idea we hardcoded the customer id in the query params. We'll work on a fix!

[remi-stripe]

@reacuna This was fixed in stripe-node 6.30.0!