https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
The server has fulfilled the request and the user agent SHOULD reset the document view which caused the request to be sent. This response is primarily intended to allow input for actions to take place via user input, followed by a clearing of the form in which the input is given so that the user can easily initiate another input action. The response MUST NOT include an entity.
原文：http://www.cnblogs.com/yuanjiangw/p/6258032.html