issue #9 提供xml消息的Builder

src/test/java/chanjarster/weixin/bean/WxXmlOutImageMessageTest.java

@@ -25,4 +25,26 @@ public class WxXmlOutImageMessageTest {
     Assert.assertEquals(m.toXml().replaceAll("\\s", ""), expected.replaceAll("\\s", ""));
   }
   
+  public void testBuild() {
+    WxXmlOutImageMessage m = WxXmlOutMessage.IMAGE().mediaId("ddfefesfsdfef").fromUser("from").touser("to").build();
+    String expected = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\"?>" 
+        + "<xml>"
+        + "<ToUserName><![CDATA[to]]></ToUserName>"
+        + "<FromUserName><![CDATA[from]]></FromUserName>"
+        + "<CreateTime>1122</CreateTime>"
+        + "<MsgType><![CDATA[image]]></MsgType>"
+        + "<Image><MediaId><![CDATA[ddfefesfsdfef]]></MediaId></Image>"
+        + "</xml>";
+    System.out.println(m.toXml());
+    Assert.assertEquals(
+              m
+                .toXml()
+                .replaceAll("\\s", "")
+                .replaceAll("<CreateTime>.*?</CreateTime>", ""), 
+              expected
+                .replaceAll("\\s", "")
+                .replaceAll("<CreateTime>.*?</CreateTime>", "")
+              );
+    
+  }
 }