Add spec for MonitorMixin#mon_initialize

* Based on #1428.

spec/ruby/library/monitor/mon_initialize_spec.rb

@@ -0,0 +1,31 @@
+require_relative '../../spec_helper'
+require 'monitor'
+
+describe "MonitorMixin#mon_initialize" do
+  it "can be called in initialize_copy to get a new Mutex and used with synchronize" do
+    cls = Class.new do
+      include MonitorMixin
+
+      def initialize(*array)
+        mon_initialize
+        @array = array
+      end
+
+      def to_a
+        synchronize { @array.dup }
+      end
+
+      def initialize_copy(other)
+        mon_initialize
+
+        synchronize do
+          @array = other.to_a
+        end
+      end
+    end
+
+    instance = cls.new(1, 2, 3)
+    copy = instance.dup
+    copy.should_not equal(instance)
+  end
+end