net/http: fix data race in benchmark
net/http: fix data race in benchmark

Fixes issue 7006

R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/44940044

pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme