dom4j 设置命名空间

附代码：

Java:

        Document document= DocumentHelper.createDocument();
        Element root = document.addElement("xml", "http://www.w3.org/1999/xhtml");
        String requestXml = document.asXML();
        System.out.println(requestXml);
```

pom.xml

        <!-- https://mvnrepository.com/artifact/org.dom4j/dom4j -->
        <dependency>
            <groupId>org.dom4j</groupId>
            <artifactId>dom4j</artifactId>
            <version>2.1.1</version>
        </dependency>

输入：

<xml xmlns="http://www.w3.org/1999/xhtml"></xml>