Fix xhtml namespace.

webmagic-saxon/src/main/java/us/codecraft/webmagic/selector/Xpath2Selector.java

@@ -75,6 +75,7 @@ public class Xpath2Selector implements Selector {
         private XPath2NamespaceContext() {
             put("fn", NamespaceConstant.FN);
             put("xslt", NamespaceConstant.XSLT);
+            put("xhtml", NamespaceConstant.XHTML);
         }
 
         @Override

webmagic-saxon/src/test/java/us/codecraft/webmagic/selector/XpathSelectorTest.java

@@ -1376,7 +1376,7 @@ public class XpathSelectorTest {
 
     @Test
     public void testXpath2Selector() {
-        Xpath2Selector xpath2Selector = new Xpath2Selector("//a/@href");
+        Xpath2Selector xpath2Selector = new Xpath2Selector("//xhtml:a/@href");
         String select = xpath2Selector.select(html);
         Assert.assertEquals("http://www.oschina.net/", select);