Inconsistent behavior between previewing subscription changes and saving subscription changes when removing subscription coupons.

stripe_subscription_coupons.rb

# == Let's work with a subscription that has an existing coupon ==

subscription = Stripe::Subscription.retrieve('sub_ABC123')
subscription.discount.coupon.id # => "early-upgrade"

# Preview invoice keeps the coupon, as expected.
Stripe::Invoice.upcoming(
  customer: subscription.customer,
  subscription: subscription.id
).discount.coupon.id # => "early-upgrade"

# == Use empty string to remove coupon ==

Stripe::Invoice.upcoming(
  customer: subscription.customer,
  subscription: subscription.id,
  coupon: ''
).discount # => nil (coupon REMOVED)

subscription.coupon = "" # => Throws ArgumentError: You cannot set coupon to an empty string.

# == Use nil to remove coupon ==

Stripe::Invoice.upcoming(
  customer: subscription.customer,
  subscription: subscription.id,
  coupon: nil
).discount.coupon.id # => "early-upgrade" (coupon NOT REMOVED)

subscription.coupon = nil # => REMOVES coupon