net/http/httputil: fix string in test failure message
net/http/httputil: fix string in test failure message

R=golang-dev, daniel.morsing
CC=golang-dev
https://codereview.appspot.com/7722043

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