Support kwargs in initializer when mixing in Concurrent::Async

Classes with keyword arguments in their initializers cannot be
initialized today if they include Concurrent::Async, as of Ruby 3.0.

Author: mlarraz

lib/concurrent-ruby/concurrent/async.rb

@@ -267,8 +267,8 @@ def self.included(base)
 
     # @!visibility private
     module ClassMethods
-      def new(*args, &block)
-        obj = original_new(*args, &block)
+      def new(*args, **kwargs, &block)
+        obj = original_new(*args, **kwargs, &block)
         obj.send(:init_synchronization)
         obj
       end