[ruby/mutex_m] Fix Mutex_m#initialize when the super's initialize has kwargs

Co-authored-by: Ryuta Kamizono <kamipo@gmail.com>

https://github.com/ruby/mutex_m/commit/84ca1fc108

lib/mutex_m.rb

@@ -113,4 +113,5 @@ module Mutex_m
     mu_initialize
     super
   end
+  ruby2_keywords(:initialize) if respond_to?(:ruby2_keywords, true)
 end

test/test_mutex_m.rb

@@ -23,4 +23,36 @@ class TestMutexM < Test::Unit::TestCase
     c.signal
     assert_equal "abc", t.value
   end
+
+  class KeywordInitializeParent
+    def initialize(x:)
+    end
+  end
+
+  class KeywordInitializeChild < KeywordInitializeParent
+    include Mutex_m
+    def initialize
+      super(x: 1)
+    end
+  end
+
+  def test_initialize_with_keyword_arg
+    assert KeywordInitializeChild.new
+  end
+
+  class NoArgInitializeParent
+    def initialize
+    end
+  end
+
+  class NoArgInitializeChild < NoArgInitializeParent
+    include Mutex_m
+    def initialize
+      super()
+    end
+  end
+
+  def test_initialize_no_args
+    assert NoArgInitializeChild.new
+  end
 end