UUIDResource has_many IdResource causes undefined method `[]` when doing GET [...]?includes=

This issue is a (choose one):

• Problem/bug report.

Description

I've tracked the issue to lib/jsonapi/processor.rb:587.

resource_set[resource_klass] is holding the associated resources in an array key'd by a string {"1" => [...]} and attempting to access it via an int resource.id.

With the following resources just issue this get request (make sure to have include=).

GET /uuids/3bbaf722-4583-4fa6-ac36-9190d4b93f80?include=ids

class UuidResource < JSONAPI::Resource
  key_type :uuid
  has_many :ids
end

class IdResource < JSONAPI::Resource
  has_one :uuid
end

Throws error:

Internal Server Error: undefined method `[]' for nil:NilClass /var/lib/gems/2.3.0/bundler/gems/jsonapi-resources-d185ebb76cd2/lib/jsonapi/processor.rb:587:in `block (2 levels) in populate_resource_set'
/var/lib/gems/2.3.0/bundler/gems/jsonapi-resources-d185ebb76cd2/lib/jsonapi/processor.rb:586:in `each'
/var/lib/gems/2.3.0/bundler/gems/jsonapi-resources-d185ebb76cd2/lib/jsonapi/processor.rb:586:in `block in populate_resource_set'
/var/lib/gems/2.3.0/bundler/gems/jsonapi-resources-d185ebb76cd2/lib/jsonapi/processor.rb:542:in `each_key'

Start at lib/jsonapi/processor.rb:587 and reproduce the failure with GET /uuids/3bbaf722-4583-4fa6-ac36-9190d4b93f80?include=ids using the UuidResource and IdResource definitions in the issue. Trace how resource_set keys are created and accessed, then verify that the request completes without the undefined method [] error and includes the associated ids.