Bugfix: include nested has_many association

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.
Merge pull request #726 from rails-api/json-api-include-with-nested-has-many

Bugfix: include nested has_many associations