Save discount amount when adding to Subscription

When adding a discount to a Braintree subscription, the amount [can be set](https://developers.braintreepayments.com/ios+ruby/sdk/server/recurring-billing/create#overriding-plan-details) to anything (which we [are already doing](https://github.com/chrishunt/fake_braintree/blob/a5203386303e069a3e665b6687b00eced6b9bc2a/spec/fake_braintree/subscription_spec.rb#L108) in the tests). Unfortunately, we're not actually saving this amount on the discount. This PR fixes that. :smiley: