`Invoiced::Invoice#payment_plan` returns the payment plan ID when it should return an `Invoiced::PaymentPlan` instance

[arthurhess]

Hi!

I'm trying to retrieve a payment plan from an Invoiced::Invoice object, but here's what I'm getting.

api_client = Invoiced::Client.new(...)
invoice = api_client.Invoice.retrieve(...)
invoice.payment_plan
# => (payment plan id, integer)

I'm guessing it's returning the attribute from @values. I might have time to investigate it later and suggest something through a PR, but it may take a while as this isn't urgent for me.

I'm working around it by instantiating Invoiced::PaymentPlan and calling retrieve on it directly:

Invoiced::PaymentPlan.new(api_client).set_endpoint_base("/invoices/#{invoice.id}").retrieve
# => (instance of Invoiced::PaymentPlan)

I wanted to leave this here anyway, in case someone else coming across this issue has time to fix it (or just use the workaround above)

Edit: I realize Invoiced::Object#set_endpoint_base is probably private API, so I'll be locking this gem's version in my project for now.