Fix incorrect s/options = {}/options ||= {}

Introduced in #965, surfaced in #1041

Author: bf4

lib/active_model/serializer/adapter/json_api.rb

@@ -16,7 +16,7 @@ def initialize(serializer, options = {})
         end
 
         def serializable_hash(options = nil)
-          options = {}
+          options ||= {}
           if serializer.respond_to?(:each)
             serializer.each do |s|
               result = self.class.new(s, @options.merge(fieldset: @fieldset)).serializable_hash(options)