serializable_hash and as_json should take options = nil

per ActiveModel::Serialization#serializable_hash
96bb004fc6/activemodel/lib/active_model/serialization.rb
    def serializable_hash(options = nil)
          options ||= {}

Otherwise, passing in nil to `as_json` or `serializable_hash`
makes things blow up when passing nil into attributes

lib/active_model/serializer/adapter.rb

@@ -16,11 +16,11 @@ module ActiveModel
         @options = options
       end
 
-      def serializable_hash(options = {})
+      def serializable_hash(options = nil)
         raise NotImplementedError, 'This is an abstract method. Should be implemented at the concrete adapter.'
       end
 
-      def as_json(options = {})
+      def as_json(options = nil)
         hash = serializable_hash(options)
         include_meta(hash) unless self.class == FlattenJson
         hash