net/http: remove two more server allocations per-request

net/http: remove two more server allocations per-request

benchmark                                   old ns/op    new ns/op    delta
BenchmarkServerFakeConnWithKeepAliveLite        11031        10689   -3.10%

benchmark                                  old allocs   new allocs    delta
BenchmarkServerFakeConnWithKeepAliveLite           23           21   -8.70%

benchmark                                   old bytes    new bytes    delta
BenchmarkServerFakeConnWithKeepAliveLite         1668         1626   -2.52%

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/8110044