Allow JSONAPI unsuffixed associations keys

As shown here :
https://github.com/json-api/json-api/blob/gh-pages/format/index.md#resource-relationships-

Author: gdlx

lib/active_model/serializer/association/has_many.rb

@@ -5,7 +5,11 @@ class HasMany < Association
         def initialize(name, *args)
           super
           @root_key = @embedded_key
-          @key ||= "#{name.to_s.singularize}_ids"
+          @key ||= case CONFIG.default_key_type
+            when :name then name.to_s.pluralize
+            else "#{name.to_s.singularize}_ids"
+          end
+
         end
 
         def serializer_class(object, _)

lib/active_model/serializer/association/has_one.rb

@@ -5,7 +5,10 @@ class HasOne < Association
         def initialize(name, *args)
           super
           @root_key = @embedded_key.to_s.pluralize
-          @key ||= "#{name}_id"
+          @key ||= case CONFIG.default_key_type
+            when :name then name.to_s.singularize
+            else "#{name}_id"
+          end
         end
 
         def serializer_class(object, options = {})