net/http: use strings.Builder

net/http: use strings.Builder

Change-Id: I754edcf21e003a3f4037fb2c5d8d06f2cd5f2fa3
Reviewed-on: https://go-review.googlesource.com/c/go/+/428267
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: xie cui <523516579@qq.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>