Added ability to set tax on upcoming invoice request

[bcrobinson]

The stripe api has the ability to set a tax rate on the upcoming invoice request
https://stripe.com/docs/api#upcoming_invoice-subscription_tax_percent

This pull request adds this property to the StripeUpcomingInvoiceOptions class.

[bcrobinson]

This is for issue #1179

[remi-stripe]

@bcrobinson Thank you so much for taking the time to submit a PR and adding a test on top of this.

I can see that another parameter is not supported yet (it was added recently). Would you mind also adding support for it: subscription_trial_from_plan. You don't need a test for that one since it's just a boolean.

[remi-stripe]

Thank you so much for the quick work @bcrobinson

r? @brandur-stripe
cc @stripe/api-libraries

[brandur-stripe]

LGTM. Thanks @bcrobinson @remi-stripe.

[brandur-stripe]

Released as 16.1.0.

[brandur-stripe]

@bcrobinson Just as a quick follow up: there's a bit of a delay on the packages because they're pushed by our CI, so the build has to complete before they're available. That said, they're generally available within 30 minutes or so of when I announce one here.