net/http: do not send leading dot in cookie domain attribute

RFC 6265 allows a leading dot in a cookie domain attribute
but is clear (see section 4.1.1) that a Set-Cookie header
should be sent without these dots.

R=bradfitz
CC=golang-dev
https://codereview.appspot.com/13111043

Committer: Brad Fitzpatrick <bradfitz@golang.org>