Add config.serializer_lookup_enabled that defaults true

lib/active_model/serializer.rb

@@ -208,6 +208,7 @@ module ActiveModel
     #   2. try again with superclass, if present
     #   3. nil
     def self.get_serializer_for(klass)
+      return nil unless config.serializer_lookup_enabled
       serializers_cache.fetch_or_store(klass) do
         # NOTE(beauby): When we drop 1.9.3 support we can lazify the map for perfs.
         serializer_class = serializer_lookup_chain_for(klass).map(&:safe_constantize).find { |x| x && x < ActiveModel::Serializer }

lib/active_model/serializer/configuration.rb

@@ -9,6 +9,7 @@ module ActiveModel
       included do |base|
         config = base.config
         config.collection_serializer = ActiveModel::Serializer::CollectionSerializer
+        config.serializer_lookup_enabled = true
 
         def config.array_serializer=(collection_serializer)
           self.collection_serializer = collection_serializer
@@ -20,7 +21,6 @@ module ActiveModel
 
         config.adapter = :attributes
         config.jsonapi_resource_type = :plural
-        config.automatic_lookup = true
       end
     end
   end