exp/cookiejar: store cookies under TLD+1 on nil public suffix list

The current implementation would store all cookies received from
any .com domain under "com" in the entries map if a nil public
suffix list is used in constructing the Jar. This is inefficient.

This CL uses the TLD+1 of the domain if the public suffix list
is nil which has two advantages:
 - It uses the entries map efficiently.
 - It prevents a host foo.com to set cookies for bar.com.
   (It may set the cookie, but it won't be returned to bar.com.)
A domain like www.british-library.uk may still set a domain
cookie for .british-library.uk in this case.

The behavior for a non-nil public suffix list is unchanged, cookies
are stored under eTLD+1 in this case.

R=nigeltao
CC=golang-dev
https://codereview.appspot.com/7312105

Committer: Nigel Tao <nigeltao@golang.org>