json: use strings.EqualFold instead of strings.ToLower.
json: use strings.EqualFold instead of strings.ToLower.

R=golang-dev, bradfitz, iant
CC=golang-dev
http://codereview.appspot.com/5127043

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