as_json not called on subclasses of Array/Hash

[bquorning]

Depending on whether Oj::Rails::Encoder is initialized with an argument or not, Oj does not call #as_json on subclasses of Array and Hash.

require "active_support"
require "oj"

Oj.optimize_rails

class ArraySubclass < Array
  def as_json(options = {})
    %w[array subclass]
  end
end

value = ArraySubclass.new
puts Oj::Rails::Encoder.new({}).encode(value)
# => ["array","subclass"]
puts Oj::Rails::Encoder.new().encode(value)
# => []

class HashSubclass < Hash
  def as_json(options = {})
    {hash: "subclass"}
  end
end

value = HashSubclass.new
puts Oj::Rails::Encoder.new({}).encode(value)
# => {"hash":"subclass"}
puts Oj::Rails::Encoder.new(nil).encode(value)
# => {}

Based on https://github.com/ohler55/oj/blob/develop/pages/Rails.md I would expect Array and Hash to be optimized, but not their subclasses.

We noticed this issue after rails/rails@6204a55 got added to Rails, and started getting everything serialized by Encoding.encode_without_option which uses an instance of Oj::Rails::Encoder created without arguments.