net/http: don't drop error on request write

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7230059
exp/cookiejar: add some more tests

New tests added for port handling and IDNA domains.
A new test case contains several redundant
tests but provides a nice documentation of the
implemented rules for domain handling.

R=nigeltao
CC=golang-dev
https://codereview.appspot.com/7393070

Committer: Nigel Tao <nigeltao@golang.org>