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/serializers/associations_test.rb

@@ -27,6 +27,7 @@ module ActiveModel
       def setup
         @author = Author.new(name: 'Steve K.')
         @author.bio = nil
+        @author.roles = []
         @post = Post.new({ title: 'New Post', body: 'Body' })
         @comment = Comment.new({ id: 1, body: 'ZOMG A COMMENT' })
         @post.comments = [@comment]
@@ -42,6 +43,7 @@ module ActiveModel
       def test_has_many
         assert_equal(
           { posts: { type: :has_many, options: { embed: :ids } },
+            roles: { type: :has_many, options: { embed: :ids } },
             bio: { type: :belongs_to, options: {} } },
           @author_serializer.class._associations
         )
@@ -52,6 +54,9 @@ module ActiveModel
           elsif name == :bio
             assert_equal({}, options)
             assert_nil serializer
+          elsif name == :roles
+            assert_equal({embed: :ids}, options)
+            assert_kind_of(ActiveModel::Serializer.config.array_serializer, serializer)
           else
             flunk "Unknown association: #{name}"
           end