Fixes nested has_many links in JSONAPI

When linked resource had has_many links, all of them would use the
association from the first resource, causing all of the items to build
`links` with the same associations.

This fixes it by iterating over the serializers, not just the
attributes array.
Is 'unless ...' needed here? If serializers is already an Array, Array(serializers) would just retunr serilizers.