net/http: use standard time formatting methods

net/http: use standard time formatting methods

time.Time has had an AppendFormat method since go1.5 so there's no
need to carry around a custom implementation.

Change-Id: I8e7e5a9ac34e8bf251f5d70555405777ce4e22a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/647955
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>