Missing two parameters for create/update add-on

[ghost]

The latest version of the SDK (2.9.0) doesn't match the latest version of the documentation.
recurly-client-php method getWriteableAttributes has the following list of fields that are allowed:
'add_on_code', 'name', 'display_quantity', 'default_quantity', 'unit_amount_in_cents', 'accounting_code', 'tax_code', 'add_on_code', 'name', 'display_quantity', 'default_quantity', 'measured_unit_id', 'usage_type', 'add_on_type', 'revenue_schedule_type'
but this should have the following two also
'optional', 'display_quantity_on_hosted_page'
These two appear in the documentation for "update-an-add-on" and "create-an-add-on"

We had to fork this, push an update and use that fork, in order to move forward with the integration.
It will be very helpful if you can update the SDK with these two parameters also.

Thanks!

[bhelx]

@mariusghitoiu Thanks for pointing this out. I'll get a PR out for this as soon as I get the chance.