Fixed a bug that appeared when json adapter serialize a nil association

[groyoh]

Take the following serializers e.g.:

AuthorPreviewSerializer = Class.new(ActiveModel::Serializer) do
  attributes :id, :name
end    
PostPreviewSerializer = Class.new(ActiveModel::Serializer) do
  attribute :id
  # note that we specify a serializer for the association
  belongs_to :author, serializer: AuthorPreviewSerializer
end

If a post is serialized with Json adapter and its author is nil, then the post would be rendered as :

{ 
  id: 43, 
  author: { id: nil, name: nil }
}

instead of

{ 
  id: 43, 
  author: nil
}

This happens only when the serializer option is set for the association.

[philipgiuliani]

Thanks a lot! I just ran into this. Would be cool to get it merged!

[joaomdmoura]

Tks @groyoh ! I'm merging it 👍