Fix another case of Hash monitor not being initialized

Author: ivoanjo

lib/concurrent/thread_safe/util/array_hash_rbx.rb

@@ -20,6 +20,12 @@ def self.allocate
             obj.send(:_mon_initialize)
             obj
           end
+
+          def self.[](*args)
+            obj = super
+            obj.send(:_mon_initialize)
+            obj
+          end
         end
 
         klass.superclass.instance_methods(false).each do |method|