cookie domain name命名规则

   A Set-Cookie from request-host y.x.foo.com for Domain=.foo.com
  would be rejected, because H is y.x and contains a dot.

  * A Set-Cookie from request-host x.foo.com for Domain=.foo.com would
  be accepted.

  * A Set-Cookie with Domain=.com or Domain=.com., will always be
  rejected, because there is no embedded dot.

  * A Set-Cookie with Domain=ajax.com will be rejected because the
  value for Domain does not begin with a dot.