cookies的默认大小限制究竟是多大。网上说是20条??4K??
RFC 上是这么说的：
- at least 300 cookies
- at least 4096 bytes per cookie (as measured by the size of the characters that comprise the cookie non-terminal in the syntax description of the Set-Cookie header)
- at least 20 cookies per unique host or domain name
不过规定是这样不同的浏览器可能又有自己的实现（就是这么任性），推送到相关问题上了解更多：http://stackoverflow.com/questions/5381526/what-are-the-current-cookie-limits-in-modern-browsers