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/adapter/json_api/has_many_embed_ids_test.rb

@@ -8,6 +8,7 @@ module ActiveModel
           def setup
             @author = Author.new(name: 'Steve K.')
             @author.bio = nil
+            @author.roles = nil
             @first_post = Post.new(id: 1, title: 'Hello!!', body: 'Hello, world!!')
             @second_post = Post.new(id: 2, title: 'New Post', body: 'Body')
             @author.posts = [@first_post, @second_post]

test/adapter/json_api/linked_test.rb

@@ -16,6 +16,7 @@ module ActiveModel
             @second_post.author = @author
             @author.posts = [@first_post, @second_post]
             @author.bio = @bio
+            @author.roles = []
             @bio.author = @author
 
             @serializer = ArraySerializer.new([@first_post, @second_post])