Improve build_json_serializer readability a bit

lib/action_controller/serialization.rb

@@ -64,11 +64,7 @@ module ActionController
     def build_json_serializer(resource, options)
       options = default_serializer_options.merge(options || {})
 
-      serializer = options.delete(:serializer)
-      serializer = ActiveModel::Serializer.serializer_for(resource) if serializer.nil?
-
-      return unless serializer
-
+      if serializer = options.fetch(:serializer, ActiveModel::Serializer.serializer_for(resource))
         options[:scope] = serialization_scope unless options.has_key?(:scope)
         options[:resource_name] = self.controller_name if resource.respond_to?(:to_ary)
 
@@ -76,3 +72,4 @@ module ActionController
       end
     end
   end
+end