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.

test/fixtures/poro.rb

@@ -40,6 +40,7 @@ Comment = Class.new(Model)
 Author = Class.new(Model)
 Bio = Class.new(Model)
 Blog = Class.new(Model)
+Role = Class.new(Model)
 
 PostSerializer = Class.new(ActiveModel::Serializer) do
   attributes :title, :body, :id
@@ -60,9 +61,16 @@ AuthorSerializer = Class.new(ActiveModel::Serializer) do
   attributes :id, :name
 
   has_many :posts, embed: :ids
+  has_many :roles, embed: :ids
   belongs_to :bio
 end
 
+RoleSerializer = Class.new(ActiveModel::Serializer) do
+  attributes :id, :name
+
+  belongs_to :author
+end
+
 BioSerializer = Class.new(ActiveModel::Serializer) do
   attributes :id, :content