Status.constructJson方法，当微博不包含缩略图时抛WeiboException

[GoogleCodeExporter]

版本：2.1.1-beta2-2

代码：
Timeline tm = new Timeline();
tm.client.setToken(accessToken);
StatusWapper status = tm.getUserTimeline();

异常栈：
Exception in thread "main" weibo4j.model.WeiboException: 
JSONObject["thumbnail_pic"] not found.:......
    at weibo4j.model.Status.constructJson(Status.java:86)
    at weibo4j.model.Status.<init>(Status.java:108)
    at weibo4j.model.Status.constructWapperStatus(Status.java:324)
    at weibo4j.Timeline.getUserTimeline(Timeline.java:190)
    at ma.weibohelper.Main.main(Main.java:46)
Caused by: org.json.JSONException: JSONObject["thumbnail_pic"] not found.
    at org.json.JSONObject.get(JSONObject.java:499)
    at org.json.JSONObject.getString(JSONObject.java:670)
    at weibo4j.model.Status.constructJson(Status.java:67)
    ... 4 more

按照微博API的描述（http://open.weibo.com/wiki/2/statuses/user_timeline��
�，thumbnail_pic应该是可选字段，不应该报错。

Original issue reported on code.google.com by maxup...@gmail.com on 2 Aug 2013 at 6:47