net/http: add BenchmarkReadRequest

Add benchmark for request parsing. Fixture data is taken from https://github.com/felixge/node-http-perf

% go version
go version devel +28966b7b2f0c Thu Feb 07 20:26:12 2013 -0800 linux/amd64

% go test -run=nil -bench=ReadRequest -benchtime=10s
PASS
BenchmarkReadRequest     2000000   9900 ns/op   61.71 MB/s   3148 B/op   32 allocs/op
ok      net/http        12.180s

R=golang-dev, bradfitz, minux.ma, haimuiba
CC=golang-dev
https://codereview.appspot.com/7313048