net/url: use strings.Index instead of a loop
net/url: use strings.Index instead of a loop

We already depend on strings in this file, so use it.

Plus strings.Index will be faster than a manual loop
once issue 3751 is finished.

R=golang-dev, khr
CC=golang-dev
https://codereview.appspot.com/8116043

wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old