Mark RubySingleThreadExecutor as a SerialExecutorService

Author: meineerde

lib/concurrent-ruby/concurrent/executor/ruby_single_thread_executor.rb

@@ -1,11 +1,13 @@
 require 'concurrent/executor/ruby_thread_pool_executor'
+require 'concurrent/executor/serial_executor_service'
 
 module Concurrent
 
   # @!macro single_thread_executor
   # @!macro abstract_executor_service_public_api
   # @!visibility private
   class RubySingleThreadExecutor < RubyThreadPoolExecutor
+    include SerialExecutorService
 
     # @!macro single_thread_executor_method_initialize
     def initialize(opts = {})