Inheritance of serializer inheriting the cache configuration

[Rodrigora]

Hi all!

This PR resolves #1219 declaring the cache related fields with class_attribute.

Please, check if the two tests cases are relevant to this change. I felt I was testing the class_attribute behavior 😕.

[bf4]

Awesome! Please change to assert_equal and I think this is good to go.

Do you think there's any value in testing all the class_attributes? I think there would be

     serializer.class_attribute :_cache
      serializer.class_attribute :_fragmented
      serializer.class_attribute :_cache_key
      serializer.class_attribute :_cache_only
      serializer.class_attribute :_cache_except
      serializer.class_attribute :_cache_options
      serializer.class_attribute :_cache_digest

I have some similar changes in bf4@a60ba9d#diff-5624f9cf35be4dc254412d979fd293c4R31 that could go along with or follow this:

+    class_attribute :_attributes, instance_writer: false
+    self._attributes ||= []
+    class_attribute :_attributes_keys, instance_writer: false, instance_reader: false
+    self._attributes_keys ||= {}
+    class_attribute :_type, instance_writer: false
#
   def self.inherited(base)
+      base._attributes = _attributes.dup
+      base._attributes_keys = _attributes_keys.dup
#
    def attributes
+      attributes = _attributes.dup

      attributes.each_with_object({}) do |name, hash|
+        if _fragmented
+          hash[name] = _fragmented.public_send(name)
+        else
+          hash[name] = send(name)
        end
      end
    end

[bf4]

#1255

[Rodrigora]

Thanks @bf4!

After working on this PR, I realized that there are many _cache_* attributes in Serializer class.
Do you think it worth to move all _cache_* attributes for other class (maybe Serializer::CacheConfiguration) for clarity?

[bf4]

possibly. There's already something like that for adapters https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializer/adapter/cached_serializer.rb