net/http: fix data race in test Fixes issue 2712.
net/http: fix data race in test
Fixes issue 2712.

R=golang-dev, dsymonds
CC=golang-dev, mpimenov
http://codereview.appspot.com/5543062

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang