Fix MonitorMixin when the super's initialize has kwargs

ext/monitor/lib/monitor.rb

@@ -220,7 +220,7 @@ module MonitorMixin
   # Use <tt>extend MonitorMixin</tt> or <tt>include MonitorMixin</tt> instead
   # of this constructor.  Have look at the examples above to understand how to
   # use this module.
-  def initialize(*args)
+  def initialize(...)
     super
     mon_initialize
   end

test/monitor/test_monitor.rb

@@ -236,6 +236,22 @@ class TestMonitor < Test::Unit::TestCase
     assert NewCondTest.new.cond.instance_variable_get(:@monitor) != nil
   end
 
+  class KeywordInitializeParent
+    def initialize(x:)
+    end
+  end
+
+  class KeywordInitializeChild < KeywordInitializeParent
+    include MonitorMixin
+    def initialize
+      super(x: 1)
+    end
+  end
+
+  def test_initialize_with_keyword_arg
+    assert KeywordInitializeChild.new
+  end
+
   def test_timedwait
     cond = @monitor.new_cond
     b = "foo"