Issue with `fields` request params

As I'm playing with the Sparse Fieldsets, I encountered some issues.

With the following resources:

class UserResource < JSONAPI::Resource
  has_one :status, class_name: 'UserStatus'
  attributes :name, :email
end
class UserStatusResource < JSONAPI::Resource
  has_one :user
  attributes :foo, :bar
end

Relationships are removed from then response when using fields, so we can't link a user to his status.

/users?include=status&fields[users]=name

Association with custom class_name failed with fields.

/users?include=status&fields[users]=name&fields[user-statuses]=foo

returns:

{:errors=>
  [{:title=>"Invalid resource",
    :detail=>"user-statuses is not a valid resource.",
    :id=>nil,
    :href=>nil,
    :code=>101,
    :path=>nil,
    :links=>nil,
    :status=>"bad_request"},
   {:title=>"Invalid resource",
    :detail=>"user-statuses is not a valid resource.",
    :id=>nil,
    :href=>nil,
    :code=>101,
    :path=>nil,
    :links=>nil,
    :status=>"bad_request"}]}

Start with the UserResource and UserStatusResource examples and reproduce the two requests using include=status and the fields parameters. Trace sparse-fieldset handling for relationships and the custom class_name resource key. Done means the relationship remains linkable and user-statuses is accepted without the reported invalid-resource errors.