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.
If type is `author` but the association is called `writer`, the linked
resource key should be called `authors`, e.g
{
...
linked: {
authors: [{
...
}]
}
...
}
Currently, doing `include: author.bio` would work correctly, but not for
has_many associations such as `include: author.roles`. This fixes it.
The problem was basically that we were not handling arrays for has_many linked,
as happens for ArraySerializers.
The options passed to the render are partitioned into adapter options
and serializer options. 'include' and 'root' are sent to the adapter,
not sure what options would go directly to serializer, but leaving this
in until I understand that better.
