net/http: make copyValues append instead of calling Add

net/http: make copyValues append instead of calling Add

This results in a performance boost:

name          old time/op    new time/op    delta
CopyValues-4    3.46µs ± 3%    1.53µs ± 3%  -55.85%  (p=0.000 n=18+19)

name          old alloc/op   new alloc/op   delta
CopyValues-4    1.52kB ± 0%    0.74kB ± 0%  -51.58%  (p=0.000 n=20+20)

name          old allocs/op  new allocs/op  delta
CopyValues-4      24.0 ± 0%      11.0 ± 0%  -54.17%  (p=0.000 n=20+20)

Fixes #33744.

Change-Id: Ibc653fb076a9a6aaa775fcc9ca720fb90e68cf96
Reviewed-on: https://go-review.googlesource.com/c/go/+/191057
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>