net/http: export Header.Clone, reduce its allocations, use it everywhere

net/http: export Header.Clone, reduce its allocations, use it everywhere

Fixes #29915

Change-Id: I6e6edf4f9a0e062211f74d120ae1a242bce1b274
Reviewed-on: https://go-review.googlesource.com/c/go/+/173658
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ingo Oeser <nightlyone@googlemail.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>