net/http/httputil: eliminate duplicate alloc/copy in ReverseProxy

net/http/httputil: eliminate duplicate alloc/copy in ReverseProxy

This commit elimates the request allocation and shallow copy duplication
already done by req.WithContext.

name         old time/op    new time/op    delta
ServeHTTP-4     216µs ±36%     212µs ±15%     ~     (p=0.853 n=10+10)

name         old alloc/op   new alloc/op   delta
ServeHTTP-4     917kB ±36%    1137kB ± 0%     ~     (p=0.352 n=10+10)

name         old allocs/op  new allocs/op  delta
ServeHTTP-4      5.00 ± 0%      4.00 ± 0%  -20.00%  (p=0.000 n=10+10)

Change-Id: I514a59c30b037c7a65c355b06fd82c2d6ff17bb0
Reviewed-on: https://go-review.googlesource.com/35569
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>