Reset ImmediateExecutor to original state.

Author: jdantonio

lib/concurrent/executor/immediate_executor.rb

@@ -1,13 +1,15 @@
-require_relative 'executor'
-
 module Concurrent
   class ImmediateExecutor
-    include Executor
 
     def post(*args, &task)
       raise ArgumentError.new('no block given') unless block_given?
       task.call(*args)
       return true
     end
+
+    def <<(task)
+      post(&task)
+      self
+    end
   end
 end