http2: remove method value allocation per read frame
http2: remove method value allocation per read frame

benchmark                        old ns/op     new ns/op     delta
BenchmarkServer_GetRequest-2     269540        259453        -3.74%

benchmark                        old allocs     new allocs     delta
BenchmarkServer_GetRequest-2     25             24             -4.00%

benchmark                        old bytes     new bytes     delta
BenchmarkServer_GetRequest-2     1613          1599          -0.87%

Change-Id: I33609d8fbe2f81e32095fac8b89ddd55592e583f
Reviewed-on: https://go-review.googlesource.com/20998
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

albertjin/golang-net,albertjin/golang-net,golang/net