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.
