Add test case for deprecated post comparison

Checks that `post_url` works for nested posts even if the path isn't specified. Also checks that a deprecation message is shown in the build log.

test/test_tags.rb

@@ -587,6 +587,32 @@ CONTENT
     end
   end
 
+  context "simple page with nested post linking and path not used in `post_url`" do
+    setup do
+      content = <<CONTENT
+---
+title: Deprecated Post linking
+---
+
+- 1 {% post_url 2008-11-21-nested %}
+CONTENT
+      create_post(content, {'permalink' => 'pretty', 'source' => source_dir, 'destination' => dest_dir, 'read_posts' => true})
+    end
+
+    should "not cause an error" do
+      refute_match(/markdown\-html\-error/, @result)
+    end
+
+    should "have the url to the \"nested\" post from 2008-11-21" do
+      assert_match %r{1\s/2008/11/21/nested/}, @result
+    end
+
+    should "throw a deprecation warning" do
+      deprecation_warning = "       Deprecation: A call to '{{ post_url 2008-11-21-nested }}' did not match a post using the new matching method of checking name (path-date-slug) equality. Please make sure that you change this tag to match the post's name exactly."
+      assert_includes Jekyll.logger.messages, deprecation_warning
+    end
+  end
+
   context "simple page with invalid post name linking" do
     should "cause an error" do
       content = <<CONTENT