XMLHTTP参数readyState和status的状态值
长整形标准http状态码，定义如下：
Number: Description
100: Continue
101: Switching protocols
200: OK
201: Created
202: Accepted
203: Non-Authoritative Information
204: No Content
205: Reset Content
206: Partial Content
300: Multiple Choices
301: Moved Permanently
302: Found
303: See Other
304: Not Modified
305: Use Proxy
307: Temporary Redirect
400: Bad Request
401: Unauthorized
402: Payment Required
403: Forbidden
404: Not Found
405: Method Not Allowed
406: Not Acceptable
407: Proxy Authentication Required
408: Request Timeout
409: Conflict
410: Gone
411: Length Required
412: Precondition Failed
413: Request Entity Too Large
414: Request-URI Too Long
415: Unsupported Media Type
416: Requested Range Not Suitable
417: Expectation Failed
500: Internal Server Error
501: Not Implemented
502: Bad Gateway
503: Service Unavailable
504: Gateway Timeout
505: HTTP Version Not Supported
readyState的状态值：
0：未初始化，对象已建立，但是尚未初始化（尚未调用open方法）；
1：初始化， 对象已建立，尚未调用send方法；
2：发送数据，send方法已调用，但是当前的状态及http头未知；
3：数据传送中，已接收部分数据，因为响应及http头不全，这时通过responseBody和responseText获取部分数据会出现错误；
4：完成，数据接收完毕,此时可以通过通过responseBody和responseText获取完整的回应数据。