net: rewrite toLower more clearly
net: rewrite toLower more clearly

Rob suggested this change.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/43670044

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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