Support foreign_type in has_one association

I am updating an app to use jsonapi:resources and have had to change a model association name without changing the underlying table. This model had a polymorphic field :resource which is now called :item, however the underlying table still has fields resource_id and resource_type.

In my model I can do this:

belongs_to :item, polymorphic: true, foreign_key: "resource_id", foreign_type: "resource_type"

However when I do this in my resource file:

has_one :item, polymorphic: true, foreign_key: 'resource_id', foreign_type: 'resource_type'

I receive an error "undefined method item_type=". From the docs it looks like foreign_key is supported but foreign_type is not. Could this be easily fixed?

Thanks for a great gem, it's been a pleasure to work with!

Start with the has_one association handling and the existing foreign_key support, then reproduce the model and resource configurations shown in the issue. Done means foreign_type is accepted without calling the undefined item_type= method, with coverage for polymorphic associations whose model name differs from the underlying type column.