public void ToStringTest()
        {
            const string _xml =
            @"<?xml version=""1.0"" encoding=""utf-8""?>
            <!DOCTYPE html PUBLIC ""-//W3C//DTD XHTML 1.0 Strict//EN"" ""http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"">
            <html lang=""ja-JP"" xml:lang=""ja-JP"" xmlns=""http://www.w3.org/1999/xhtml"">
            <head>
            <meta http-equiv=""content-type"" content=""text/html; charset=utf-8"" />
            <meta http-equiv=""content-style-type"" content=""text/css; charset=utf-8"" />
            <title>title1</title>
            <style type=""text/css""> </style>
            </head>
            <body />
            </html>";

            var html = new XHtml();
            Assert.AreEqual(_xml, html.ToString());
        }