Support billing_cycle_anchor in Subscription Update

[adrianlungu]

Right now, the Subscription Update API doesn't seem to support billing_cycle_anchor as it throws the following:

localstripe_1  | TypeError: _update() got an unexpected keyword argument 'billing_cycle_anchor'

I'm not sure how complex the support for this might be, however, as changing the billing_cycle would in theory also create a new invoice and attempt a payment ?

[adrienverge]

I'm not sure either, but I would say it's complex. Subscriptions (and invoices) lifecycle are the most complicated parts of localstripe.