Extract context build to a method

lib/action_controller/serialization.rb

@@ -65,11 +65,21 @@ module ActionController
       options = default_serializer_options.merge(options)
 
       if serializer = options.fetch(:serializer, ActiveModel::Serializer.serializer_for(resource))
-        options[:context] = { scope: serialization_scope }.merge!(options[:context] || {})
+        options[:context] = build_context_from options[:context]
         options[:resource_name] = controller_name if resource.respond_to?(:to_ary)
 
         serializer.new(resource, options)
       end
     end
+
+    def build_context_from(context_option)
+      default_context.tap do |context|
+        context.merge! context_option if context_option
+      end
+    end
+
+    def default_context
+      { scope: serialization_scope }
+    end
   end
 end