修复使用<code>XmlDocument</code>加载含有DOCTYPE的Xml时，加载后增加“[]”字符的错误

        public static void FixDocumentType(this XmlDocument xml)
        {
            if (xml?.DocumentType == null)
                return;
            var name = xml.DocumentType.Name;
            var publicId = xml.DocumentType.PublicId;
            var systemId = xml.DocumentType.SystemId;
            var parent = xml.DocumentType.ParentNode;
            var documentTypeWithNullInternalSubset = xml.CreateDocumentType(name, publicId, systemId, null);
            if (parent == null || xml.DocumentType == null)
                return;
            parent.ReplaceChild(documentTypeWithNullInternalSubset, xml.DocumentType);
        }