net/http: send correct time in Date header.

Date header indicated that it contained GMT time,
however it actually sent local time. Fixed by
converting time to UTC.

Also fixes incorrect comment in appendTime().

Regression since CL 9432046.

R=golang-dev, dave, bradfitz
CC=golang-dev
https://codereview.appspot.com/13386047

Committer: Brad Fitzpatrick <bradfitz@golang.org>